I played through unmodded vanilla BG 1+2 several times.
Now I installed BGT and I have just finished the BG1 part.
I found these differences:

- BGT removed the exp limit of BG1. I had about 210000exp when I beat Sarevok.
I dual Imoen to mage before I met Yoshimo at level 11(You need a thief in the first part of the dungeon). She left the dungeon as lv4 mage and I hope she can use both classes when I get her back.
- New weapon profiencies. But I had no problems with it. Khalid uses longsword, not bastard sword for example.
- I removed the exp you get when you learn a spell, pick a lock or disarm a trap by changing a file. Without it I would have gained a level before leaving candlekeep.
- In BG1 you can stack ammo up to 20. In BG2 it was more, but now I have the limit of 20 in BG2 too. You can buy stacks of 40 in a shop, but you cannot refill unless it gets below 20.
- There are 2 wolfweres/werewolfs you can only harm with silver weapons. There are only 4 of them in the game, 2 are bastards swords, for a longsword you have to finish durlags tower and the last one is a dagger. I tried to kill him several times without success and then I found another bug. A greater wolfwere increases his healing rate every time you reload, and I had to reload a lot. Finally I had to kill them via console.
- In the highest level of durlags tower is a succubus. In vanilla I got a quest from her if I approach her with a female char. In BGT I had to fight her.
- When changing from BG1 to BG2 the reputation number has been reset, but not the chars. So Minsc was happy and Jaheira complained as soon as I got them out of their cages.
- In Irenicus Dungeon you can find items you got from BG1. Usually those are baldurans helmet and the armor "protector of the second". I found the helmet as usual, but in the room with the dwarfs I found : Full Plate+1, gauntlets of ogre power, wand of the heavens, ring of holyness, cloak of balduran, gauntlets of dexterity, ring of fire resistance, ring of free action, cloak of protection+2, gauntlets of weapon expertise, ring of wizardry, shadow armor, boots of stealth, bracers of archery, cloak of non detection. All of these items were equipped when I finished BG1.
- BGT removed the "respawn bug". In vanilla BG1 was a problem. When you kill one enemy of a group and you save (and maybe reload) a new group will spawn. When I found this bug, I had to kill a giant army of goblins and wolfs after I tried to kill one after the other. Some enemies still respawn in BGT, but it is much better than vanilla.
- Some things changed in BG2 as well. But I have also installed "unfinished business" and maybe something else, so I am not sure if this comes from BGT.

summary: BGT makes the game much easier (except the werewolf bug where you may have to cheat to continue if you don´t know it). I will finish the game I started until the end of ToB. But if I ever play it again, I use vanilla BG1.

BGT is a brilliant mod, brilliantly implemented. Is it perfect? No. But it's a mega mod, and no minor mod is perfect, let alone mega mods. What BGT suffers with is the same problem that TuTu suffers with; it's not just the graphics that's 'improved', but everything else 'BG2' that comes with the conversion. I love BG1 vanilla above all the BGs, but I still applaud the BGT team.

I agree that the BGT team did a great job.
But it makes the game much easier.
Especially if you do not remove the exp from scrolls/locks/traps as I did.

By the way:
I found out that you can steal many quest rewards before you even start the quest.
But I do not know I this was already in the vanilla game.
It is the first time I tried to steal from everyone who has a name (not just commoner, peasant, nobleman or something like this).

Edit:
Lets see if another bug still exist.
In vanilla BG2 there was a bug: When I start a fight in one room and I leave this room (a new room is loaded, but you do not save or need to gather your party) the enemy follows you but when you beat him and go back, he is still there. This way I could have gotten the celestrial fury infinite times, if I wanted to.

The bug did not appear on BGT baldurans ship when I separated the wolfweres by luring them to another level of the ship.

Agreed. This is only true when the reward is part of the NPC's inventory, and marked as 'removable'. Some rewards are given by script, in which case they cannot be stolen. This must be a bug in your setup. It should not be the case that the originals are still in the previous room. I have never come across this.

Umm...that sounds like something specific to you: I was able to stack ammo up to 40 in both games. Maybe I installed a tweak from the tweak pack which did that? If so, there's a fairly easy fix to that problem at least.

You must have installed a tweak. The limit is 20 in vanilla BG1, and 40 in vanilla BG2.

To be clear, I was talking about playing the two games with BGT installed.

Oh, I see. I haven't played BGT in quite a while, but presuming it doesn't alter (BG2) stack levels by default -- I can't remember offhand -- that would indeed make it 40 in both games.

You touch on what I consider to be the biggest problem with BGT there -- that it's merged into one long game. NPCs have, at least in some cases, different attributes in BG1 and BG2, but this is ignored by playing it via BGT. Therefore I much prefer to use Tutu.

And how do you reconcile the illogical differences with some character stats? The stats you refer to have no consistency between the two games, and that is why they are altered. You can use the tweak pack to set the stats to BG1 standard, BG2 standard or no change. It's not an issue.

They are different between the games, hence I want to play them the way the developers intended. Which makes the way BGT deals with this an issue.

I also prefer to have two games behaving like two games, so I much prefer Tutu over BGT. For others that won't be much of an issue, but for me it is.

The 'way the developers intended' is a contentious issue when you consider that they were changed at all. But to each his own.

It's how the games were released, so it's a pretty safe bet to say they were intended to be like that...

But just in case it wasn't clear enough... IN MY OPINION Tutu is preferable due to above mentioned reasons. You clearly feel differently, which is fine by me. I play the games how I like, and you how you like.

It's a pretty safe bet to say that they thought the first stats were wrong, or inadequate in some way, or...
You get the picture. I never said that TuTu is preferable. That is your assumption. I merely pointed out that you don't have to play BGT with altered stats, if that's what you want.