Before I start nitpicking, let me say that so far I've rather enjoyed BG III. The funny thing is that even if I knew that BG III had to start fresh, and therefore didn't expect that it could connect to BG and BG II except for the setting, I was still a bit disappointed that it didn't. Go figure. ;-)
I've seen a lot of good feedback already, so I'm not going to repeat it. So I'm going to focus on things I haven't seen so far, which of course doesn't mean they haven't been commented on already.
Encounter design/skill checks: It wasn't surprising to me that if I blow my skill checks in my dialogues, I would end up in combat. What did surprise me is that it lead to instant death or the character being disabled and unable to participate. One example is the encounter with the mind flayer after the ship has crashed, but the worst offender is the winery encounter. If you missed an encounter before you went to Waukeen's Rest and entered the winery, and then missed one of your two skill checks, you're dead, and there is absolutely nothing you can do about it. As an ex-DM I'd say an encounter like this is known as messing up your players. Things like this actively discourage exploring, because either you start blowing up everything unknown in sight, or you simply use quicksave about every thirty seconds. Neither is very fulfilling.
There is already a lot of good feedback on the skill checks in the forum and I wholeheartedly concur.
Encounter design part 2: So I go into the underdark, see that strange mist and ( I forgot who) someone says "Here be Myconoids!" Fair enough, so I carefully go ahead, only to be jumped by two Minotaurs. Myconoids and Minotaurs both start with M, but apart from that I don't see much of a relationship. Not only that, but before I can even react, Shadowheart is down (jumped and gored), and Gale's hp severely depleted (gored again). And that was just one of the two. I prevailed in the fight with three dead members, but still. where did that come from? It may be realistic, but a) where did that bloody Minotaur encounter come from ( I didn't see any others in my underdark journey), and b) wtf with so many strong attacks? i was shortly reminded of WOWs Burning Cursade: "You are not prepared." This is the worst offender, but there are a few others, e.g. bloated hyenas you can't kill off, but have to wait until they transform into gnolls. Or enemies that don't mind setting everything ablaze, resulting in killing off half their party with friendly fire.
Companions: If I'm being realistic, I'd kick almost all party members. Gale's been eating away the besst loot I find, Shadowheart is following Shar and trying to be a poster child for arrogance, Astarion is a bloody vampire ('nuf said), Lae'zel is an obnoxious, overbearing bitch with a superiority complex. I may need help, but in reality I probably wouldn't be that desperate. I haven't really found out what is wrong with Wyll, though he also seems to have some dark secret. My point is that, yes, I do want the companions have some sort of interesting background, but I found this a bit too much.
Combat with enemies on high ground: I have no problems with enemies seeking the high ground, and that they have an advantage in attacking my party. What I have an issue with is that currently it is extremely annoying targeting those enemies, if you can target them at all. This is more an engine issue than everything else, as the engine apparently tries to display your party, and therefore cuts out structures that are in the way. If that structure is the one your enemy is standing on, you're out of luck. Waiting out those enemies or getting out of LoS doesn't seem to help, as they
display some cleaver behavior and stay in place.
Character editor: When I first created my character, I was truly pleased with the editor. It didn't go overboard with the choices, but they were varied enough, and hey, you can actually get decent hair (are you listening, Bioware?). After playing the game, my feelings are a lot more mixed. There are a lot of choices that can really influence how your playing, and me not being a DD 5e buff simply didn't understand how my choices in the editor actually does that. My understanding has grown, but there are still quite a few areas where I have no clue how things work out. Since Larian decided to go as closely with %e as possible, the editor should give you the option to guide you through your choices. The current help texts are not really helping.