Cessium.
I almost never use it.
Seriously, Skills, Feats, and weirdness of the bonus system (luck, circumstance etc.)
Here is how I would "fix" the bonus system:
I would make stacking based on object location (no more than 2 rings), and what game stat it modifies. Thus- anything that increases AC that is not armor, you can only use one of at a time. Anything that increases strength, saves, a skill etc you can only use one of at a time. Thus you can't get a circumstance AND a luck bonus to climb. If you have a item that gives +2 to Str and a separate item that give +1 the first takes precedence and all you get is a +2. Still, you can wear both, and when the creature that eats +2 Str magic items eats it, the +1 kicks in. Spells count for this too. A spell that gives you +4 str takes precedence over the item that gives +2.
This makes effects things much much easier to track and manage.