A day/night cycle, for one. It's not glaringly absent right now because the only hub is the grove and the rest is wilderness, but i wonder how Baldur's Gate will turn out without it. Can't think of many games that did cities well, or indeed just did cities, without passage of time. It just makes them feel more alive all around.
Wouldn't call it a mechanic more so aesthetic thing. If day/night cycle impacts gameplay in some way beyond npc spawning/despawning, like improving stealth, reducing vision to non darkvision enemies, then i would consider it a mechanic.
