Worst case scenarios are really important in game design whenever you have a struggle between people. DnD 3.0 was a great example of this. Fighters had lots of ways to increase their Armour Class like putting on plate, getting a natural armour necklace, wearing a deflection ring, putting on a shield, and getting magical and mithral versions of their armour and shield. The problem was that if they did all this many monsters that were supposed to be a 'reasonable challenge' simply couldn't hit the fighter. For the average case the Armour Class system worked fine, but for the worst case it was a disaster.
You can see the same sort of problem in WOW. Everybody goes around hit capped such that they can never miss on an attack. This isn't a problem. The problem came when people got their avoidance so high that monsters actually couldn't land a blow on them. There were plenty of funny videos around during Burning Crusade of people soloing raid bosses by having 100% chance to dodge. Thankfully it didn't end up really wrecking any raid content but it required some heavy handed kludging by Blizzard to avoid that.
Because of this it is important to keep bonuses to Armour Class low and keep randomness high. As long as everybody is rolling a d20 to hit and a reasonable amount of those numbers will connect everything is fine. It might not feel realistic that a veteran soldier layered in magical protections can still be hurt by some dork but it keeps the degenerate case from happening. I took this to heart in skyRPG and tightly controlled access to abilities that increased character defenses. You can get tough, but you can never get yourself to the point where enemies are utterly unable to hurt you. Problem is, I followed the same logic with Skills and that was a big mistake.
How often should a random dude like me be able to jump further than an olympic calibre jumper? Never! 1 in a million when the olympian trips and falls halfway through their run maybe? Unfortunately in DnD the 1d20 mechanic ensures that I beat the olympian 5% of the time. Raw Strength checks are even sillier. The strongest man in the world adds 5 to their 1d20 rolls and I add 0. So if there is a heavy object that I might be able to lift the strongest man in the world still usually fails to lift it? Preposterous.
The difficulty is that the system for success with skills has way too much randomness in it. When any fool can succeed at a check with a couple tries it hardly makes any sense that someone who is a master of the trade might not make it. What skyRPG needs is a skill system that places more importance on the skill of the character and less on the randomness of the die. I figure the simplest way to fix this issue is to just shrink the die. If I am attacking someone I roll 1d20 + Dexterity, but if I am balancing on a ledge I roll 1d8 + Dexterity + Acrobatics. This way you end up with a system where the basic mechanic is still die roll + stat but combat stays safely random and non combat makes some semblance of sense.
Pics from: http://www.jimnolt.com/firstencounters.htm and http://blogs.indiewire.com/shadowandact/jamie-nieto-olympic-high-jumper-has-acting-to-fall-back-on-or-jump-into