There's little things about every game that bug me.
I've learned to ignore these things, because I have to.
If I'm writing for someone else's game, there's no point in trying to reinvent the wheel if something is small, but also fundamental.
Here's an example: attacks of opportunity.
I love the idea that certain circumstances will trigger free attacks.
I hate the list of things in a typical d20 rule set that trigger those free attacks.
Let's look at these shall we?
This seems like a good idea on the surface. You shouldn't just be able to walk around someone in combat. Orderly retreats should be difficult. Opponents fleeing in bad order should be cut down.
Except of course, that it leads to people charging an opponent, then standing completely still while they make a full attack every round.
Worse, the way to avoid it is just dumb. You mean if I take my 5-foot step first, there's no AoO for me fleeing the Mind Flayer in terror at x5 my run speed, but if I take the move then the 5-ft. step he kills me? Huh.
So wait, it's bad enough that he sundered my sword, leaving me with my 1d2 punch vs. his +3 sword, but he also gets a free attack? Why is that?
Ummmm.... uhhh.... reach! Yeah! He has a reach advantage!
Huh. So if a Hill Giant and I step into the squared circle for a little unarmed pay-per-view, does he get a AoO against me for reach? No?
And of course, as any 16th century Italian fencer will tell you, a small weapon in close will beat a large weapon every time. But the AoO rules don't represent that.
So it's not bad enough my AC sucks to holy hell, and that if I fireball this dude right in front of me I'll be blowing myself up, but he also gets a free attack while I cast my spell? Why is that?
Ummm, uhhh, because you can't dodge and cast a spell!
So casting a spell takes more concentration than avoiding another guy's sword, avoiding his shield, finding the weak spot in his armor, and making a thrust?
As you can tell, I don't buy any of this crap.
It's not necessarily bad design mechanically. It's just dumb. If casting a spell triggers an AoO, swinging a sword should.
If getting hit with a sword stops a spell, it should stop a sword swing.
If movement always caused an AoO, except for a Fighter ability to allow "withdrawing in good order" or a thief ability to "spring attack" or "wall walk", then I could get on board.
If the person with the smaller weapon always incurred an attack of opportunity, but always threatened a critical when he made it inside his opponent's defenses to deliver that punch or that knife thrust, then I could get on board.
It doesn't do any of those things though.
AoO are a good idea implemented in a really bad way. They add no excitement and add a lot of "bog" to the game.