A lot of variety.
An entertaining game, both in terms of gameplay, and visuals. Everything was nicely animated, and cartoony to the point of being visually fun without being juvenile. The types of unit were varied enough to keep things interesting, and to cater to many different strategies.
The game auto-saves, which is nice, but not-quite-so-nice is the sheer appalling length of even the first level. It takes forever to play through, and you may wind up getting demolished in the ending waves of the level. (Perhaps a faster setting than 3x would help, here.) Oh yeah, the quit button is absolutely unneccessary.
Overall, it's a well-balanced and interesting tower defence game, with great replay value to boot.