This is what bothers me the most, because its a simple programming issue to do stat check before randomizing finds.

Encumberment/weight systems.

It's trying to make things more "realistic" by completely arbitrary and unrealistic means.

I'm fine with stuff where only what you've actually got equipped counts towards encumberment like in From games. But it's just particularly annoying in cases where if you have your inventory say more than half full by even .1 it impedes your movement to the level of a petrified sloth with lead ankle weights and makes the game unplayable.

Not to mention that it comes up in games with random loot systems, which dilute the value of equipment since you end up with a bajillion pieces of equipment with slightly different plusses or elements or what have you and it can be hard to sort out what's actually worth keeping and making keeping backup weapons worthwhile. (Again I prefer From's approach to loot, with more limited but useful and there's often a story behind how it got to where it is.)

Yep.

Maybe you could search for a mod that translates it all to "X to Y" damage. While I prefer the regular X to Y damage (honestly though, X damage is even better) it wasn't that hard to learn. Just find a good guide before you start playing so you don't have to switch back and forth, interupting immersion.

I mean, now I have some idea of how to read a stat format like that, and I'm sure I'll look for a guide the next time I play a game with that stat format, but it's still an unnecessary hurdle that requires completely unnecessary work for anyone who either is completely unfamiliar or only vaguely familiar with the setup.

Statistics that don't give a rubric for understanding them. For example, if I may choose a dagger that has 5.60 attack vs. a dagger with an attack of only 4.40, but which also gives a +4 strength bonus ... what the heck does that mean? Is +4 strength good enough to make up for lower stats?

"Go for the eyes, Boo, go for the eyes!"

inventory weight 100 (I can run effortlessly)
inventory weight 100.1 (way to much to even move)

I call it Morrowind!
Not to mention that Line you've heard a million times about said emperor, After a while it got quite Maddeningly DERANGING!
You've been playing Oblivion again!

I guess you just played the wrong games implementing it then. In Mass Effect series... Yeah, the changes are minor. But then there's Witcher 2 which switches around the entire second act (as in, the ENTIRE second act, including vast majority of NPCs, locations etc.) based on your decisions. Or Alpha Protocol that allows you to make major changes to dialogue and NPCs that you're going to encounter. Or... Well, there's bunch actually. I do agree that most of games implementing the system don't do very good job of it - but that applies to any system, ever

Means you are stronger so your hits are 4 times harder you can also carry More Items!

It's been mentioned already, but what bugs me the most in some RPGs is that the damage you inflict or receive is based on your level instead of your stats. Eg: At lvl 20, you attack a creature but hardly do any damage, but at lvl.21 (without an increase in stats), you can suddenly kill the damn thing in 2-3 hits because 'its level is less than 3 levels higher than yours'.

Hmmm....

When plot-specific doors require keys. In a game that you have fireballs, thieves with an endless arsenal of lockpicks, ridiculously strong warriors with large weapons and freaking teleport spells.

I hate it when the ending of the series makes all the choices you made while playing the series irrelevant. You know what I'm talking about. :D