Having played a bit longer, I've arrived at a few thoughts on surfaces. First, people saying they aren't a part of 5e isn't exactly correct. If a party wants to, they can hurl oil all over the place (and caltrops and ball bearings). But the effect of those things is much more diminished in 5e than it is in BG3 as it currently stands. Here is how a flask of oil is described in the PHB:
Oil usually comes in a clay flask that holds 1 pint. As an action, you can splash the oil in this flask onto a creature within 5 feet of you or throw it up to 20 feet, shattering it on impact. Make a ranged attack against a target creature or object, treating the oil as an improvised weapon. On a hit, the target is covered in oil. If the target takes any fire damage before the oil dries (after 1 minute), the target takes an additional 5 fire damage from the burning oil. You can also pour a flask of oil on the ground to cover a 5-foot-square area, provided that the surface is level. If lit, the oil burns for 2 rounds and deals 5 fire damage to any creature that enters the area or ends its turn in the area. A creature can take this damage only once per turn.
It's an even 5 damage, but note that a creature can only take this damage once per turn. Most spells effects (like Wall of Fire or Moonbeam) work the same way. Some (like Spike Growth) inflict the damage for every 5 feet you move. If the damage (and specifically, how often it ticks) were toned down, they would bother me much less, and would feel less abusable.
Continuing on that, here is the description for ball bearings:
As an action, you can spill these tiny metal balls from their pouch to cover a level, square area that is 10 feet on a side. A creature moving across the covered area must succeed on a DC 10 Dexterity saving throw or fall prone. A creature moving through the area at half speed doesn't need to make the save.
Ice works in a similar manner.
Second, barrels being movable is fine, but you shouldn't be able to put them in your inventory. Where are the characters storing barrels that likely weigh more than them and are 3 or 4 feet tall? I know that inventory in video games is often unrealistic in this way, but I'd like to see BG3 move away from it.
Best post about surfaces so far.
I think this really is a topic where both Larian can get their surfaces and players can get their DnD 5e rules, without much compromise.
Why did Larian combine Firebolt and Create Bonfire? We have one spell that creates a small fire surface in the rules, let's use that. It's these little attentions to the rules that get's people who want to play 5e the video game really excited.