The only real Issue that is most likely perception based on my part is the perceived tendency of the game to have damage rolls that fall short of expectations. This appears to happen more frequently with the higher levels of difficulty. I'll use Magic Missile as a prime example, 2-5 damage per missile 3 missiles average damage should be 9 with a min of 6 max of 15. In explorer mode that's what you get, (over 300hrs playing between EA and live) you kick it up to mid level overall the average stays the same BUT... when it comes down to that mob that has 9 hp left you'll do 8. or throw out a spell that should do say 3-24 like Thunderwave and between save and low dmg rolls I have more often than not seen damage in the low single digits.
Or on the flip side of that (receiving end) I'm going to use the Gnolls in Act 1 as an example, they will use their ability for rapid attacks (can't remember the name ATM) and they will hit 2 of 3 times with damage on the high end of the possible every time they hit. I try something similar (potion of speed +Bless + Divine Favor) I get the floating :Miss: :Miss: :Miss:
This is the kind of stuff that happens in this game that annoys players so much. More often than not over an entire battle it averages out. But with the streaks we get, the possible seems to pop up more often than the probable. So far the Gnolls seem to have the outliers more often than say Goblins but any battle can go pear shaped quickly with just a few outliers thrown into a battle.
TBH the perception is that these outliers (really high or low) happen more often in favor of the mobs, that is the issue behind all these threads. But people forget that those critical hits from their Paladin with Divine Smite that kills the boss in one hit is an outlier too.