There was a lot of newness and creativity in the Bard's Tale games, and I could not wait for the newer versions to come out. Unlike Gregz I believe the games got better as the games went along.
I thought BT2 was mostly an incline: even more devious level design, the puzzles which I actually liked, and introduction of range in combat making things more tactical. On the downside the bestiary was rather silly; each and every dungeon level had it's own set of monsters with mostly silly names. In BT1 it was easier to "relate" to the monsters.
The developers had do to do something to ramp up the challenge for new players and players importing alike. Players like me took the same party from BT1 -> BT2 -> BT3. I believe that since you could finish BT1 then continue leveling, they had to scale the difficulty upward in each future game. So BT2 added real time puzzles. Finish the puzzle or everyone dies.
Personally I found BT2 to be too easy if importing characters from BT1. I remember quitting halfway on my first attempt, and restarting from scratch. That was a much better and more challenging experience, and the beginner dungeon was excellent.
BT3 added a lot of creatures with all kinds of death and crit abilities..
BT1 and BT2 already had monsters with various nasty special attacks. But due to bugs those special attacks were not present in the Amiga and DOS versions of BT3. For me, playing the Amiga version, BT3 was a huge disappointment, partly due to this bug. BT3 also had a weird mix of real time and turn based gameplay. Encounters were finally no longer checked in real time, but mana regeneration was. So you could regenerate mana by just having your characters hang around and not move.
So BT3 remains the only BT i haven't completed.
Incidentally I think it says something that BT2 and BT3 are among the few games the crpgaddict did not complete.