It gets even more difficult as different multiplicators for the magnitude of changes had to be taken into account. For example, a change to the armor mechanic would be huge (and I wish Larian would have changed the stupid DnD AC system which is perhaps necessary for PnP but an insult to mind on PC, as better systems are easily found and could be calculated), while a change to some monster's hp or abilities at least for me is neglectable even if I put myself in the position of a DnD fan, as the changes don't seem to be massive in most cases. Gameplay-wise it is no difference whether we got a changed DnD monster or a new monster with the same stats.
I'm not saying that all changes are bad, but many changes on monsters are huge and not neglectable gameplay-wise.
- I find the changes to phase spiders absolutely terrible. It makes combats against them slower than they should and/or nearly 100% ranged oriented. The "Dancing Light" spell as implemented doesn't help, especially if all your characters doesn't have nightvision.
- It's exactly the same with harpies and considering their area design, it's even slower because you're often lower than them. It makes our characters run a lot more than they would if the harpies had proper DnD actions.
- Minotaurs now have an AoE jump that allow them to disengage and to reach our weaker characters easily (So is their charge if I'm not wrong). Melee characters should be able to protect them or deal damages if minotaurs rushes your second lines... they aren't.
- Goblins and many other creatures can shove as a bonus action... which become a part of many combats for various reasons (push you of course - which may lead to insta death or a lot of additionnal movements - waking sleeping characters up, disengage)
- Many creatures also have more HP than they should, making the "sleep" spell less usefull.
- Some creatures also have higher ability score, which mean in exemple that their ST are better than they should. This can really mean something when you cast a spell.
I guess we could also talk about the massive amount of consumables that gives extra actions to many ennemies but it would probably lead us a bit too far.
That's just a few exemples. TBH as a non-DnD player I haven't gone so far into analysing this but a lot of changes aren't neglectable whatever we don't care, we like them or not.
Of course, it wouldn't change anything gameplay-wise if the "phase spiders" of BG3 were called "spitting spiders".
But I guess most of us can easily understand the dissapointment when something you like for years is suddenly changed. Only DnD fans care about the "iconic DnD creatures" so if you want to satisfy the fans, you create something that corresponds to what the fans are fans of and not a different version of it with the same name... On top of that many DnD creatures are better gameplay-wise than Larian's creatures as stated above. Personnal opinion though.
