This was discussed ad nauseam back in March around TD2 release.
Visually TD2 might be better, although that is arguable since TD1 supports SLI and TD2 does not. Other than that, TD1 all the way for me.
More than anything, TD2 character movement is what I hated most after the initial bugs and glass walls issues were reduced. The character runs like they have a load in their backpack and in their diaper.
One of the things that really annoyed me and I despised was the way that TD2 progresses the World Tiers without any warning with no way to return. I had leveled up 4 characters all staggered by about 4-5 levels so that I could pass the gear down from one to next, then discard. But, by the time I hit end game, I was so disenfranchised that the WT level up was the last straw. I haven't played it since. I had stuff I wanted to do, explore more, etc. In TD1 the open world is kind of empty at the end game. In TD2, they went overboard the other way. It was slightly better than the enemy spawn type and rate in TD1's West Side Piers area, but only slightly.
I think if had started with TD2, I probably would have liked it fine. But, they had such a great foundation for this series. They could have taken TD1, swapped locations, added some missions, new enemy factions, and perhaps a few new skills and they'd been golden. For myself my game play tells it best. over 4k hours in TD1, ~130 hours in TD2 (I think).
Thing that puts me off with TD2 the most is the movement - the change in TD1 after 1.8 was bad enough but in TD2 I was constantly feeling like someone was nudging my arm when it came to the movement and so often stepping past where I wanted to go, etc due to the artificial restrictions on the movement. I understand the issue they were trying to solve with the change but it makes no sense to me - why I got into TD1 originally was because the movement was so fluid and responsive when so many games lately are horridly clunky so the change significantly detracts from the experience for me. (Also I can only assume it was only tested in a low ping environment as when I'm playing on my 4G, etc. which is perfectly capable of gaming, rather than my broadband, it is far far worse moving around and far more often hitting the turn cap and ending up with odd movement).
Also not a fan of many of the mechanic changes - TD1 got a lot of them right IMO just needed to be refined and/or implemented with the final vision for them from the ground up - too much was changed around for changes sake IMO and/or due to a small number of high profile streamers who seem to have influenced the mechanics towards what works to their advantage (PVP) with their play style specifically.