Defense Grid is a good TD game to me, which means I can usually manage about one level a week or so and then I get bored again. It seems to err more on the side of playability rather than having the perfect build order, which I appreciate. What bugs me is that, at the top end of difficulty at least, every one of these games (that I've seen) turn into a paint-by-numbers. Mathematically, in order to get the furthest you have to build two of tower A, then three of tower B, then upgrade one of A and one of B, twice, then build tower D, etc. etc. Anyone just going into it and tossing down towers where they like and hoping for the best is going to fail. Anyone who builds an extra tower when they're meant to upgrade but follows the rest of the build order exactly isn't going to get as far. It's almost like it's trying to be a pure puzzle game, instead of a hybrid action game. But if you fail you have to start not just the most recent puzzle again, you have to start the entire game over again. Bleh. Also the slippery slope of not having enough or the right type of firepower so the waves push through too far, so your firepower is distracted, so you don't get a good start on the next wave, and so you keep falling further behind...
Anyway, I'm going to stop ranting, but I really just have little patience for TD games. I don't know why I feel that way, I don't think I've seen anyone else like that, but they really bug me.