I generaly prefer when games give XP for getting things done rather then kills (Deus Ex, Pillars of Eternity) - completing quest, exploration and such. Giving XP for individual feats (kill, disarming traps, succesful conversation rolls) teases the powergamer inside me. How much XP I get shouldn't be influenced by how I complete the task or get to the area, and I shouldn'y be encouraged to kill every enemy, open every door and disarm every trap if I have no need for it (outside XP gain). Trying to reward with XP multiple playstyles (like modern Deus Exes) only amplifies the problem. Still, there are players who hate not being numerically rewarded for the activities. Either way, it doesn't change my playings, but knowing there is a way to milk more XP annoys me.
The experience that players were gaining from kills was extremely small, something like 3 exp per goblin at level 3. Level 3-4 normally takes 2700 experience, or 900 goblins.
Well, first of all this was a demo. We don't know if things like XP gain are tuned to be near final. For example, XP gain could have been tuned down as to not accidently trigger level up [speculating].
Even if 3 exp would be accurate, it is unknown if BG3 will follow the same lvl up thresholds.