In Heroes By Trade I have a system for determining which powers characters can use. One of the basic stats determines the character's Vigour, which in turn determines what rank of powers they can use. The maximum rank usable is Vigour + 6, so a normal character with Vigour of 5 can use at most rank 11 powers. Normally they won't want to do this though because on each following turn they need to roll Vigour +1d6 and they can only use a new power if the result of the roll is at least as high as the rank of the last power used.
Example: I have Vigour 5. I use the Blizzard power which has rank 11. Next turn I roll 1d6 + 5 and get 9. 9 is not as high as 11 so I have to use a basic attack and try rolling again each turn until I succeed.
Example: I have Vigour 5. I use the Zap power which has rank 6. Next turn I automatically can use another power because 1d6+5 is always 6 or more.
What this means is that characters have to roll 1d6 each turn and remember what power they used last. Mechanically I love the way it plays out because people consider whether or not to use a low rank power and automatically make their roll or use a high level power and take some downtime. It adds a lot of strategy, particularly later on when characters have a large arsenal of powers to choose from. The trouble though is that the players constantly forget to roll their 1d6 or forget which power they used. It isn't complicated to figure out but it does seem clunky in implementation.
There are other ways to approach this. I could for example have Vigour determine the maximum rank of power a character can use but just let the characters use those powers any time they please without any rolling. This has the unfortunate side effect that low rank powers would almost never get used because there is no longer any penalty for using a max rank power. It is very simple - either you have enough Vigour to cast Fireball or you don't - but it loses a lot of depth.
The last method I have been considering is one where before a character uses a power they roll 1d6+Vigour to see if it works. If the roll fails there are two possibilities: Perhaps the character just loses their action entirely or maybe they have to default to a basic attack or defence. Either way I suspect that people would find it very frustrating to position themselves for a big kaboom and then have it fail.
I love the current implementation theoretically but I don't know that everyone playing buys in completely. I guess I build the games I want to play; for me remembering what I used last and to make a roll is super easy but some folks don't have the same viewpoint. Can't make a game for everyone!