This is not a TCG (trading card game) and that's what makes it so great.
There is no card collection or deck building, each game you get a random selection of 12 basic cards out of the game's pool of 48 basic cards and 4 special cards from the class of your choosing.
"So what if I get completely screwed over and only get bad cards?" - That doesn't happen because a) the game is incredibly balanced, every card has its use and strength b) the selection isn't totally random, the game makes sure that you have 2 healing cards every game and that it's a fair mix of low and high cost cards ad that you always have board clear cards.
So to get back on point what makes the lack of deck-building great is that you have to adapt to your cards every game. Do i try to get an advantage by being more mana-efficient than my opponent, do I try to win via board control or do I just rush my opponent? If you want to win you have to play to your decks strength. Also a big part of the game is reading your opponent and predicting what cards he has.
One example: I'm ahead in board control, my opponent just played a very low impact card and he has a lot of fire mana saved up, that means that he is probably preparing to play Armageddon which deals a lot of damage to all creature on the board. So if I correctly predict this I can take advantage of it by not overcommitting and not playing any expensive creatures next turn because they'd probably die.
Advanced players will use this to bluff you. e.g. a player might save up a lot of air mana which makes the opponent afraid of the spell Lightning Bolt, which deals massive damage to the enemy player's hp. So the opponent may waste his turn by casting expensive heals even though it was a bluff and the opponent doesn't have Lightning Bolt.
All this predicting and bluffing wouldn't work as well if the game didn't have a limited number of cards.
Oh I almost forgot the most important point about not being a TCG, the game isn't pay to win, it's all about skill.
Other important points:
- The game has still an active online community even though the game is 5 years old
- The computer AI is really clever and good at predicting your moves, so if you don't like online play, it's never boring to play against the computer
- The campaign is pretty okay, every fight has special rules, so it's nice and varied, though as I said the best thing about the game is the online play
- you can get a lot of value out of the demo. I played like a hundred online games before I bought the game (but a year or so ago they added a 5 games per day restriction to the demo)