The thing that really annoys me about all tower games I've played is that the targetting AI is lame - imho it should always be the farthest advanced unit that is targetted.
I agree that it's lame. I think it should be something like the enemy closest to your base with a preference for the ones with lower life. You may not be able to kill the closest one in time, but the one behind him you could kill if your towers would just shoot at it.
If I were ever to make a tower defense style game, I'd try really hard to make the towers a little smarter.