I am about half-way through the campaign of Prime World: Defenders. It is tower defense with collectible card game elements, which sounds like a game designed for me. Tycho at Penny Arcade has similar gaming tastes, so his recommendation convinced me to pre-order before a really good sale. And hey, pre-ordering gives you a little sale plus some little bonuses.
If you like tower defense, you will like Prime World Defenders. It has minimal mazing elements, mostly choosing which towers to build and (slightly) upgrade along fixed paths. Enemies and towers have the standard mix of air/ground, fast/slow, groups & splash, stealth, healing, etc. You have spells (1 until you buy talents) and no hero unit; this is not action tower defense like Orcs Must Die! or Dungeon Defenders. You can buy talents to improve overall effectiveness.
The collectible card mechanics are the non-standard element. It operates at three levels. First, towers and spells are cards. You get the basic cards as you progress, and there are rarer cards to be found. Having found relatively few, I do not have an indication that rarer towers are absolutely better, but they can be upgraded further. Upgrades are the second level: you can recycle cards to level up other cards, and artifact cards have no purpose except to recycle for larger values. Evolution is the third level: you can recycle duplicates to level up a card in a slightly different way, which unlocks the ability to upgrade the tower on a map. Towers therefore have two level counters: towers can become permanently stronger, and then within each map you can pay to make each a level 2 or 3 tower.
This advancement is both good and bad. To me, in terms of quality of play, it is on balance bad. It makes balance difficult, because the same difficulty does not work for fully upgraded towers and fresh-from-the-pack cards. My one fully upgraded tower minces enemies, and I cannot imagine how devastating a fully upgraded rare tower must be (unless that balance went badly in another direction and it is not worth finding and upgrading them). It is hard to get a satisfying challenge under these circumstances; you do not know if you have good strategy or just good numbers. Difficulty is so far aimed low, so you can complete the levels despite poor strategy or low numbers, and you can go back later to complete them with no leakage and/or either achievement. I like that this also allows you to play around a bit more instead of looking for the One Strategy that meets highly tuned difficulty.
If that is good grind for you, you will love this. There is procedurally generated content so you can get some variety in map layout and enemy composition, and you could spent weeks collecting and upgrading towers. If you would prefer to skip leveling and have all the content tuned for the level cap, Defense Grid will probably be more to your liking.