This article is mostly for newer players. People who have been playing for a while likely already know everything I'm about to say. But for new players, this advice could help you speed up your learning process.
When I first started playing magic in 93 or 94, I knew next to nothing about which cards were good and which were bad. All I knew was a card that had a high power and toughness could stomp things well. I also knew I hated Royal Assassin. When I went to college, I lost track of the game, and then March of 2011, I found the game again thanks to two coworkers. I picked up right where I left off, thinkin that big creatures were the bomb. Every creature I played was near vanilla, and even though I played decks that used cards from the begining of the game till recent, I couldn't win more than a few games against my friends. My decks were too slow, and too cumbersome even when I played with a full set of mana vaults a sol ring, a farseek, and metamorphosis. With that list i could get out an big stomper fairly quickly, but Craw Giant died almost every time before he had a chance to attack. Then I would be left with no creatures and a game loss.
Fast forward to now, and I'm starting to really get a feel for this game. I'm still making mistakes in judgement, but at least I'm not thinking nettletooth djinn should be a high dollar card cause juzam djinn is the same card with just one more power and toughness. So how do we look through the list of cards we are presented with every few months and figure out which cards will be awesome, and which will be just so so or worse? You can either wait and read articles and have somebody tell you what is good and what is junk, or you can learn to look at cards like they do and find the gold among the pyrite.
You need to look at every card and ask it a few questions to see if it is worth your time or not. The first question you should as is "When I call upon you, what will you do for me and how soon will you do it?" There are a ton of cards out there with cool abilities, yet they see next to no play for the single reason that they act too slow. If a card has a really cool ability but can't use it till your next turn due to summoning sickness, then the card fails this test. Lets look at a card to see a great ability that fails this test. somberwald sage, Holy Mana Ramp Batman! This card has to see play right??? You wish for that, but the card has too many downsides to see regular play in anything other than specialized decks. The black lotus ability is great even if it only works for creatures, but the body is weak, and worse still, you tie up 3 mana for a creature that does nothing to influence the game the moment it comes out. Giving your opponent time to react to your moves is what makes this card go from "could have been great" to "mehh." If you give your opponet a turn to react, they you are giving your opponent a gift. If our sage had haste, then it would be 10X better. Not only would it be in essence a free creature, but it would allow you to cast other creatures of colors your land can't produce. It would be the ultimate birds of paradise with haste. Now lets look at a creature that does do something for you the moment it comes into play. Everybody should be familiar with restoration angel. This card is great because the moment you cast it, it is doing you a favor 99% of the time. It has what we call an immediate board presence. Not only is the body on this creature good for a 4 CMC, but it can protect your other creatures, and trigger their enter the battlefield abilities. Havin an ability that influences the board the second a creature is cast is probably the biggest thing in determining if a creature is any good or not. If they need to go grab a cup of coffee before they do anything, they likely need to get fired.
Next, you ask yourself "why are cards like delver of secrets and tarmagoyf are so popular and good"? This leads you to the second question you should ask a card if it answers NO to the first question. "Is this card such a bang for the buck that it is worth waiting for"? Cards like our little insectile aberration and the goyf obviously answer yes. The reason they answer yes is because you get so much for so little and even when they don't work out, you didn't extend yourself much trying to get them. If you get a relatively big creature for next to free, even if it has no special abilities, it is a good deal. Lets face it, other than the size of tarmagoyf he does nothing. Dispite this, he is a $90 card. His size isn't even that big unless you get a variety of stuff dumped into all the graveyards in play. If all graves are clear, the goyf is only a pathetic 0/1 for 1G. That is an absolutely horrible creature. Likewise delver of secrets is a horrible creature as a 1/1 for U. But something happens to these creatures either immediately if you are the goyf, or hopefully on your next turn if you are Delver. These two creatures get bigger. In Legacy, graveyards fill up fast, and the Goyf goes from a 0/1 to up to an 8/9 for the price 1G. An 8/9 creature that can be chump blocked all day is no big deal and should be passed upon if you are paying 8CMC for it. But at 2CMC it becomes the deal of the century. It is like buying a Ferrari for the price of a Dacia 1310. Tons of respect to anybody who knows what a Dacia 1310 is. Like me when I started playing this game, I though the Dacia of creatures was great cause at least it had 4 wheels and a seat. But now that I am older, I need something more to impress the ladies (don't tell my wife). Like our goyf, Delver also gets perks under the right condition. His flip side insectile abberation has a "casting cost" of reveal info about an instant or sorcery you just put in your hand. For that tiny cost, you change your 1/1 vanilla into a 3/2 flier. 1CMC for something as respectable as a 3/2 flier is great! Normally in blue you would have to pay 4 or more mana for a creature that nice. So what Delver really says on the card is Save yourself 3 or more mana and likely get your 3/2 flying creature out sooner by just waiting a turn for this card to grow. Delver and Tarmagoyf are litterally the chia pets of magic. Saving mana is a huge deal in magic, and is why both delver of secrets and tarmagoyf are both creatures that span formats from standard to legacy. If you can get the same board presence out as your opponent, yet do it for less mana, you are winning the game because the mana you saved can then be spent to assault your opponent. UW delver is such a great deck because most things it does it does well for so little mana. Those of us who play delver decks should feel lucky we got him as a common. If he were instead a rare or mythic rare, he would be a decently expensive card.
So, is Slumbering Dragon as good as Delver or Tarmagoyf? For the cost of R, you get an immediate 3/3 flying creature. Unfortunately that creature can't do squat. So it certainly fails the fist test of "what do you do for me the moment I call upon you" test. Does it fail our second test of "are you worth the wait?" For R, when he is finally worth having, he is an 8/8 flying dragon. So initially he seems to be worth the wait. But how long do you have to wait for him to go from sleeping worthless dragon to 8/8 dragon of doom? Looking at the current meta, UW delver is common. Lets play this exchange out and see if Delver Vs the Dragon is good for you. I'll give you a hint, it isn't if you are the dragon. Turn 1 both players play their 1 CMC creatures. Your opponent gets a 1/1 delver, and you get a 3/3 flying dragon. Turn 2, your opponent flips the delver and gets a 3/2 flying insect and hits you for 3. You are now down to 17 life but your dragon is now a 4/4 flying sleeping dragon. Next turn same as the 2nd. You are now down to 14. This pattern continues on till you have an 8/8 dragon that finally can block the incoming delver, but your life total is already down to 8 with 3 on the way. So as you then declare this monster as a blocker, your opponent says vapor snag and you go from 8 to 4 life. 2 more turns and it's game over. Now obviously most games won't be as booring as that. You will have created other creatures, and your delver playing foe will have likely created other creatures too. And if you are playing around your dragon, you likely have some spells in hand that will give him counters quicker. So it would likely be online sooner than what I depicted. But unlike Delver and tarmagoyf, the dragon needs other cards specifically in your deck to make him wake up. So you should really think of his casting cost as R + (insert token generating spell/ability here). You could use blessing of Nature but it as a CMC of 5 if you are not top decking it. As a result, in this case our dragon would cost 4GR + getting attacked for your 8/8 creature. 6 CMC for an 8/8 flying creature isn't BAD, but it isn't GREAT considering you need 2 cards to make it happen, and you likely have to wait a few turns to pull it off. Certainly the sleeping dragon could end up costing you only GR + getting attacked if you pull of a miracle, but we shouldn't be relying on miracles to win us the game unless we have library manipulation that can help ensure we have on top of the library what we want on top.
In this light, slumbering dragon just doesn't meet the "are you worth the wait" standard. He certainly could be if you can influence the wait time with a Miracle. But his usual CMC is 5+, and stacking your library with cards that limit your options. Playing 4 slumbering dragons, 4 blessing of nature, and 4 increasing savagery takes up too many slots in your limited deck that it makes the dragon a creature to likely pass on. He is only worth it if your entire deck revolves around creatures with counters on them. So if you do have a counter hungry deck, by all means, play the dragon, but the dragon is no delver of secrets or tarmagoyf. He is a niche card where the goyf and delver fit in many many decks.
Anyway, there are other questions you should ask a card when determining if it is worth playing or not, but my fingers will fall off if I keep typing. I will leave you with one more bit of advice. READ! Read every article you can about magic then pick it apart and learn from it what is worth learning. An education is key to success in most every thing we do in life. If you want a high paying job, you need an education or a skill very few people have. A mechanic isn't just a guy who owns a wrench and can change the oil on a car. He/she is an educated person (hopefully), who has spent years studying how to fix cars like the amazing Dacia 1310. A mechanic knows how to analize the car/card and determine if it is worth fixing/using.