@etonbears

Yes I definitely agree that circling around an enemy should be made more difficult, given the extreme bonus you get for doing so. This could be addressed by reducing the bonus (numerical +2/+3 bonus), requiring actual flanking (allies on opposite sides of an enemy), or having such movement provoke AoOs. Or many other solutions.

Re: Faerie Fire, Reckless Attack, Vicious Mockery
I agree with you that these options all still have uses. The enemy could climb up to you, in which case having mocked/faerie'd them the previous turn would have been a good decision. Those examples you gave are good. I suppose I was being hyperbolic when I said these were completely invalidated, so my bad.
But the effects of them definitely are lessened, as you mention in your post (for reckless attack and faerie fire at least).

As I said, I do like the idea of getting a bonus from high ground. And I'm not opposed to some kind of flanking/backstab bonus. I just want the reward-to-risk/cost ratio to be more balanced and negatively affect fewer spells/abilities. Currently, high ground&backstab are so easy to get and provide such a strong bonus.
I think this would be best achieved by changing it from Advantage to a stacking numerical bonus and/or changing backstabbing to flanking.

@Niara
+1