It's a problem if it makes the game unfun
This one is always a problem as the purpose of playing the game is having fun. That said, it depends on how one defines "unfun". Impediments and setbacks may not be directly fun, but assessing risk and making the gamble (and paying the price when the gambit fails) is fun.
It's a problem if it makes the game unplayable
Much like the first, it's hard to play and have fun if the game is unworkable. GMs can apply apply enough force to make things happen "appropriately" despite the rules, but I find that is a big problem.
It's a problem if it makes the game severly unbalanced
This becomes a problem if either the player all notice and take advantage of the imbalance -- which tends to create non-genre appropriate groups -- or if only a few players notice and then dominate play. A GM can try to rebalance on the fly through a variety of techniques, but it is a problem for me.
It's a problem if it makes the game moderately unbalanced
As above, but it is a lesser problem that is more easily balanced by applying filters to the game such as encounter construction, antagonist creation, and deliberately sharing spotlight.
It's a problem if it makes the game slightly unbalanced
Barely a problem as no system with a lot of options is perfectly balanced.
It's a problem if it makes the game less fun, less balanced or less playable than an alternative
This is less a probalem than an opportuity for improvement -- if one provides enough critical forethought to verify the alternative is actually preferable including any ripple effects across the rest of the game engine.
It's a problem if it's chosen to often because of it's power
It's a problem if it's chsoen to often
It doesn't matter why something is chosen; power is just a very common reason to choose a game ability. If it is chosen enough to be considered a non-choice, it can be a problem in that it narrows design space.
It's a problem if it creates too much work for the DM
This can be very much a problem. Persuading others to DM is hard enough. Extra work is one of the turn offs. Increasing that workload will act as further disincentive. Additionally, if the extra work is at the table, the DM has a greater chance to screw up in the moment which makes everyone feel bad.
It's a problem if I can't "make sense" of some mechanic within the fiction
I find this one to be some small problem for me. If I can't make sense of the mechanic either through a genre lens or by positioning inside the game universe then I always wonder why it exists and if it should be replaced.
It's a problem if it's too complex
Complexity is only a problem if it is unneeded and unwanted. If the mechanic is central to a thesis and deserves complexity, it is a feature not a bug. If the mechanic is tertiary and is expected to be glossed over during play, complexity is a problem.
It's a problem if it's too simple
The opposite of complexity, really. There are appropriate times for simplicity and times where a weightier alternative is more appropriate.
This one is always a problem as the purpose of playing the game is having fun. That said, it depends on how one defines "unfun". Impediments and setbacks may not be directly fun, but assessing risk and making the gamble (and paying the price when the gambit fails) is fun.
It's a problem if it makes the game unplayable
Much like the first, it's hard to play and have fun if the game is unworkable. GMs can apply apply enough force to make things happen "appropriately" despite the rules, but I find that is a big problem.
It's a problem if it makes the game severly unbalanced
This becomes a problem if either the player all notice and take advantage of the imbalance -- which tends to create non-genre appropriate groups -- or if only a few players notice and then dominate play. A GM can try to rebalance on the fly through a variety of techniques, but it is a problem for me.
It's a problem if it makes the game moderately unbalanced
As above, but it is a lesser problem that is more easily balanced by applying filters to the game such as encounter construction, antagonist creation, and deliberately sharing spotlight.
It's a problem if it makes the game slightly unbalanced
Barely a problem as no system with a lot of options is perfectly balanced.
It's a problem if it makes the game less fun, less balanced or less playable than an alternative
This is less a probalem than an opportuity for improvement -- if one provides enough critical forethought to verify the alternative is actually preferable including any ripple effects across the rest of the game engine.
It's a problem if it's chosen to often because of it's power
It's a problem if it's chsoen to often
It doesn't matter why something is chosen; power is just a very common reason to choose a game ability. If it is chosen enough to be considered a non-choice, it can be a problem in that it narrows design space.
It's a problem if it creates too much work for the DM
This can be very much a problem. Persuading others to DM is hard enough. Extra work is one of the turn offs. Increasing that workload will act as further disincentive. Additionally, if the extra work is at the table, the DM has a greater chance to screw up in the moment which makes everyone feel bad.
It's a problem if I can't "make sense" of some mechanic within the fiction
I find this one to be some small problem for me. If I can't make sense of the mechanic either through a genre lens or by positioning inside the game universe then I always wonder why it exists and if it should be replaced.
It's a problem if it's too complex
Complexity is only a problem if it is unneeded and unwanted. If the mechanic is central to a thesis and deserves complexity, it is a feature not a bug. If the mechanic is tertiary and is expected to be glossed over during play, complexity is a problem.
It's a problem if it's too simple
The opposite of complexity, really. There are appropriate times for simplicity and times where a weightier alternative is more appropriate.