Well. For starters, they changed the players' scripts. (default behavior). Even your own, handmade characters, do stupid things completely on their own. You have mages who charge towards the enemy as front liners, puny dagger in hand, trying to cut them down, and popping up in tiny pieces a second later.
They altered some cutscenes. Others, are replaced entirely, but by completely irrelevant or new ones that make no sense.
It feels more cartoonish, some cutscenes especially like intro with Sarevok, lack motion and gore.
In 1 enhanced, you can get ALL spells in scrolls, available for memorization. In 1 original, some were missing, and were exclusives to npcs spellbooks, like Wraithform.
In enhanced 1, you can exceed the XP cap, by playing the black pits adventure. It is interesting, but a hard challenge. The only thing i really liked in the enhanced version.
They restored some cut-off content. Don't know what exactly; some areas, some quests...
Added 3 npcs that can be recruited. Some gays in the official game forums, demand implemented romance path for male protagonists, with the male Half Orc Blackguard, one of those characters... Thankfully not all communities are like GoG, listening closely to their fans' desires...
Enhanced 1 has the mechanics of BG2 original. Which translates to watered down players. You can no more summon vast armies of creatures from summon spells, the weapon proficiencies are chaotic (in 1 investing a few pips on one category, gave you mastery over MANY weapon types, plus 1's system was more beneficial in general), thieves in 1 enhanced are crippled (because they get all the bunch of 2's skills to spend points on, so they cannot max all the key ones as they could in 1 original), spells that were very potent in 1 original (like horror, web, and hold person lasting longer and getting resisted difficult) now work as in 2 (watered down; but in 2 you are a powerhouse anyway, and the spells key in 1 are somewhat useless in 2), you can no longer create a general class in 1EE and choose a kit in 2EE (once you make in 1 enhanced a character, you cannot change it in any way upon transition to 2, except only for ranger racial enemy) which causes many powergaming AND ropleplaying issues (for example in 1 i would use a ranger wielding Drizzt's equipment with scimitar and shield, and in 2 i would turn him into a beastmaster with staff and sling; or create a druid in 1 to get higher stats and turn him into avenger in 2, so as to bypass the 16 to str and sta restrictions of the kit), and the list will reach plus infinite if i don't arbitrarily end it like now. Long story short, enhanced edition players, be them npcs or handmade, are not as strong and/or exploitable, as they had always been throughout the original saga. Keep that in mind.
You can no longer use Firebead Elvenhair to level up in 1. They "fixed" him. You can no longer use the tanner in 2 to hit max level. They seem to have ostracized ALL xp loopholes.
The spells in 2 EE no longer reek of cheese. You can no more utilize simulacrum to create like 12 copies of your main, each one casting his own planetar, and all the army happily pulverize each and every boss that would otherwise stomp on your dead carcass flat (hello, Demogorgon, dragon bhaalchild, etc). No more vampiric touch and larloch's return stolen xp when used on coffins and containers. It's all a bugger. 2's real charm has always been the pretty imbalanced spellcasting. Take this away, you took 2's real charm away. Congrats!
Finally, if you use xp loopholes in 2 original, you can end up with more weapon proficiency points than normally allowed, on dual classed players, by not levelling up until you exceed your inactive class with your new one; which means no weapon prof pips wasted on your second class, save for when choosing it. As you can imagine, this is no longer the case in the EEs, with the loopholes and stuff removed. I like my 13/28 berserker/mage with 3 pips to dual wield, and grandmastery to BOTH katana and dagger, thank you very much.
Ah, and if by any chance you expected those nice, fancy, premade characters that the original games featured, those nice little "bags" crammed full of endgame items you could "borrow" on your newly created character (by starting a multiplayer game and exporting your character next), just forget it. Also, forget the Tales of the Sword Coast saved game, which you could load and start an expansion only playthrough with the premade party of Abdel and co. They removed that, too. Especially since you could "borrow" their items on your newly created character, from there, too.
Me? I do not like the EEs the least bit. I am diehard original BG. I bought them only because GoG brought them in, because i am a collector (want entire, complete series on my shelf with nothing missing from any given franchise), and because it was on a serious discount (50%), so i couldn't resist. Hopefully, i will discover glitches and exploits along the way, to help me STOMACH the EEs and play them, at least for one full play. And i found a couple, already. Like importing your ToB character to 1EE. Or finishing properly 1 original, import/convert characters to 2 original, and then import to 1 EE for a complete, 1+2 EE playthrough. Lets see...