I think some of the disagreement here may be language - I think everyone agrees someone on high ground has an advantage, and someone on low ground has a disadvantage - the questions is; should someone on high ground have "Advantage," and someone on low ground have "Disadvantage?" I would argue, No.
Advantage is equivalent to +5, or a 25% increased chance to hit. Same with Disadvantage in the negative. A +5 is the same as 3/4 cover. So, at an equivalent distance shooting at someone above you, in the open, is equally hard as if they were at your elevation with 3/4 cover - I'm just not seeing it.
I agree some modifier is appropriate, but would prefer to see a +2/-2 (same as 1/2 cover), over full Advantage/Disadvantage. As it is now, it just seems to make one combat strategy far to powerful, and, IMHO, makes combats a bit boring; get high ground and ping away until dead. Move to next target.
