oh man I've sunk hundreds of hours into Defense Grid and threw them some money for the expansion/sequel Kickstarter. Admittedly, I do enjoy the TD genre and I'm a bit of a completionist for this one in particular. Sanctum is more of a hybrid because of the active FPS aspect. DG is more passive and traditional. While competent, Sanctum to me lacked soul, even though the upgrade system is deeper. For more active tower defense, I had more silly fun with Orcs Must Die solo and Dungeon Defenders in co-op (didn't try OMD2 yet. Oddly, I'm not sure I want more of that same, even though it'd probably be my fave TD co-op if I played it first instead of the others). Newer TD games may be innovating, but DG is still my go-to.
Defense Grid's presentation is nice and I like the checkpoint/rewind system (but no in-level saves), the UI is a little consoley. If you liked the demo, the full game is a just lot more of the same gameplay, but you really shouldn't expect any surprises other than map variety. The real reason I've played it so much is because there's a bunch of challenge modes for all the maps and getting gold stars on all of them requires different tactics, so lots of experimenting instead of relying on the same old build order. Steam Achievements, Cloud saves, Leaderboards (I like duelling scores with my brother) makes it inviting to pick up even a year later. There's probably no more content in the pipes for DG1 now that DG2 was just announced, but in the time that it's been out, the devs have already released extra levels and modes for free as well as cheap DLC packs (a dollar for a pair of maps with like 5 modes each, and that's before sales). Defense Grid is easily in my top 3 casual time killers. FTL is creeping up there though.
Actually, my brother came over to watch NBA playoffs today, but as soon as that's done, I'm going to make him try another map so I can heckle him from the couch