This is my new favorite Tower Defense game. (Sorry, Rinku.) If you have ever enjoyed a TD, you owe it to yourself to play this one.
One of the biggest draws of this game is its adjustable difficulty. You can pick settings for each level before you go into it -- things like 50% more creeps per wave, 20% more HP, bonus armor -- and each setting has a multiplier attached to it. Each level has a base XP value, which is modified by the combined multiplier of your settings and by any in-level amulets (basically achievements) that you earn. So the harder you make it, the more XP you gain.
As you level up, you get more skill points, which you can allocate to things like starting mana (money), increased mana gains, and bonuses for each individual tower type. This means that your capabilities rise over time, and you can go back to earlier levels to increase your score. Insane replayability (on an already massive game)!
But that's all meta to the actual game. What makes it a good TD? Well, the biggest thing for me is that there are so many things to do with the gems. You can put them into towers and tower-defend just like you'd expect, but you can also A) put them into traps, decreasing their damage but amplifying their special effects; B) put them into amplifiers, augmenting nearby towers and traps; C) throw them directly onto enemies as bombs; D) use them as fuel for shrines, which provide powerful attacks on a long cooldown; and E) mix & match them for extra power.
I could keep going on here, but if I haven't convinced you to try the game yet, another few paragraphs aren't gonna help :)