Are there differences in terms of quests, changes of the gameplay?
I know some NPCs were added, but is there something that has been removed?
The new companions correspond to some of the new class choices that are available. The Blackguard kit for Paladin, the Monk (base class, but implemented interally as a Thief variant), and Wild Mage kit for the Wizard. There are also the Sorceror and Barbarian classes, and a few other kits that BG1 didn't have.
There are some changes to the user-interface to scale it to larger resolutions, and mouse-wheel or pinch can zoom in or out the scaling.
Some changes that will also be evident from comparing Baldur's Gate 2 to Baldur's Gate, because they are doing the same as the Tutu mods and running a fixed-up version of the Baldur's Gate 2 version of the Infinity Engine to run the original game: http://www.pocketplane.net/tutu -- two items of note here, aside from the above
I believe the team wanted to implement more new races but weren't allowed to by Atari or Wizards (not sure whom), but you get Half-Orc addition at least.
Various mods out there fixed issues with the original game. A short list from this forum post explains which ones are no longer needed: http://forum.baldursgate.com/discussion/6967/bgee-mod-compatibility-thread/p1
ToBEx fixes game engine bugs by modifying the executable: http://www.shsforums.net/topic/43639-bg2tob-tobex-release-thread/ To be honest, I'm having trouble weeding out what here is just hacks/mods and what's actually been fixed. I believe a scroll-bar on the kit selection UI means you can add more kits to the game without not being able to select the last added ones (something modders had to deal with by removing other kits).
BG2 Fixpack: http://www.gibberlings3.net/bg2fixpack/docs_core.php Fixes to descriptions, sounds, barbarian/berserker rage/enrage, and a ton more.
Dudleyville fixed over 700 things: http://www.forgottenwars.com/dudleyfix/index.htm Some of which are fixes to errors in NPCs (illegal proficiencies for classes, for example), spells (actually work like the description, imagine that), etc.
One Pixel Productions list of changes are: http://www.spellholdstudios.net/ie/1pp
Edit: I just realized there's one actual removal-- there is no longer EAX support for position audio and reverb. It's a little disappointing but not a deal-breaker, IMO.