If you find something, please let me know. I'm the exact same way. I LOVE Defense Grid, I LOVE Plants vs. Zombies and have been looking for something similar too.
I think PixelJunk Monsters might have been the closest I've come, but that's a PSN exclusive. You could also check http://www.towerdefence.net/
and see if something there works, but I don't recall finding a clear winner when I checked that site out.