One thing that was a big part of BG1 that's pretty much absent from BG2 is wilderness exploration. There's none of that wandering around numerous outdoor areas, unnamed on your map, that you get in BG1. You start in a city and spend much the game there, when you travel elsewhere you generally head straight to a defined location, and any wilderness areas are in the vicinity of such places and are much more limited. Whether that's good or bad is a matter of opinion; for my part it's the one respect in which I consider BG1 better (though I consider BG2 better overall), but you may disagree or not mind either way, and there are still lots
of places to see and things to do in BG2.
The above and the difference in character levels are the two biggest differences between the two games.
In BG1 there is so much more wandering around the wilds. If you aren't familiar with the game, then checking all 4 sides of every (outdoor) map to find the adjacent areas is generally necessary. BG2 has less of these areas with large, empty (YMMV) spaces. In BG1, you're running around exploring the world, while in BG2 you're running around doing things for people (or possibly looking for more people to do things for).
BG2 NPCs have significantly more dialogue while in your party, and they'll chat with each other as well. This is mostly absent from BG1.
Higher levels mean stronger abilities, both used by and against you.
Edit: I enjoyed BG2 more. Part of that might be that I just have less time, but I think it is also because I didn't enjoy the wandering-around-exploring-everything-just-because-I-can aspect as much.
Edit 2: I played BG2 years before BG1. There are throw backs, but it is perfectly playable without playing the first one.