User blog comment:PrayforDawn/What is Difficulty?/@comment-2010873-20140113102943

IMO "room for error" is the best general measurement of difficulty. The more skillshots and micromanagement (of energy, ability stacks, cooldowns, timing, etc.) that a champion has in executing their combo or using abilities, the more difficult it is to use that champion since there is more potential for mistakes.

I agree with everything you said except regarding mobility, so I'll focus on just that..

I don't think mobility "is the number 1 factor in determining difficulty," or even necessarily raises the difficulty of a champion. In fact, I think mobility LOWERS difficulty.

Examples: 1. A fairly difficult champion where mobility lowers difficulty: Ahri. She's reasonably difficult not because of her mobility, but because of how critical it is for her to land Charm in her combo to maximize her damage or expose key enemy targets. Missing Charm, which happens often because it is a rather slow skillshot, means you lose a massive 20% damage bonus and valuable CC. Thankfully, Spirit Rush, her main source of mobility, repositions herself to favorably land Charm and escape whether or not she misses it, lowering her difficulty. 2. A difficult champion where mobility is not the determining factor in difficulty: Lee Sin. He's one of the most mobile champions in the game, so positioning is a priority, thereby raising the difficulty, but this isn't the main reason. The difficulty lies in the enormous potential for his combos/ganks to go wrong. Miss your Q, which happens often because it's a skillshot, and you have little potential to even start your combo. Failure to basic attack between abilities to regain energy can mean lost DPS. Kicking the enemy in an unfavorable direction could mean an enemy kill lost. Kicked the wrong enemy in a team fight (like an enemy Amumu into the middle of your team)? Congrats, you just threw the game. Not to mention the difficulty of timely ward-hopping to maximize his utility. The list goes on. 3. A fairly easy champion where mobility lowers difficulty: Kassadin. He is a rather easy champion to play (after level 6) due to his massive mobility and the ease of pulling off his combo. His Q is targeted, and silences his target so they can't respond and disrupt his combo. His E has a large hitbox, so you can't miss with it. Most importantly, his R is a Flash with low cooldown that gives him superior mobility to dive and escape.

Imagine how much more difficult it would be to use these champions and others if it weren't for the mobility to help them set up their combos. Why is Flash such a popular summoner spell? It's the mobility that it offers, allowing a champion to set up combos or make abilities more effective (Sona's Crescendo or Shen's taunt) through instant repositioning. Indeed, mobility makes it easier for a champion's potential to be maximized, hence lowering their difficulty.