There's a post on the Supers! board about problem powers, and since I can't get to the board from work, I thought I'd pontificate here.
My first thought, deliberately contrarian, is, "What problem powers?"
In one sense, the player chose it and you agreed to it, so there shouldn't be a problem. An attitude of letting everyone be excellent at something goes a long way toward this—everybody playing wants to be cool at some point, and they should. It's a superhero game, not a game of existential failure. So, yeah, in an ideal world, there would be no problem powers, and you see that in games where the powers are purely narrative.
At least, in some games all powers are equally the same amount of problem—the powers can be limited by the fictional reality more than the game mechanics (or the game mechanics refer to the fictional reality, I guess). Marvel Heroic Roleplaying approaches that kind of game, I think. So does Masks.
But it's not an ideal world. For Supers! and ICONS and Mutants & Masterminds and DC Heroes and so on, there can certainly be problem powers.
I think that problem powers are rare, but if you're in a problem situation, it won't seem that way. Problem powers are really a combination of things, because for some groups or some players, these problems aren't problems at all. It's an intersection of player, GM, character, and the way the power is written.
It seems to me, though, that the problems happen when this power wrecks the fun of players (including the GM). Some powers make things too easy in a way that the GM didn't anticipate; some powers destroy niche protection so that no other character is special; some powers have consequences that are just mind-blowing. This was suggested in the Supers! thread: what if a character with time travel wants to pop back in time to the beginning of the fight and change the conditions? I doubt you planned the fight with that in mind.
Off-hand, I can think of the following problem powers, in no particular order.
Time Control. Usually the base form (speed yourself up or slow others down) isn't too bad...it becomes a pain when you introduce time travel into the mix. What if someone wants to pop back two combat rounds and help himself? What a book-keeping pain.
Telepathy. It's really difficult to run any kind of mystery when the character can casually walk in and read minds. "Ah, Ozymandias killed the Comedian. Concentrate your search there."
Wizardry. How do you preserve any kind of niche protection when one character can do anything?
It's true too that some powers are fun-sucking when taken to the logical extreme. There's a lovely piece out there about how control of feces would be an incredible super power. Or the ability to speed up time on others is horrific if you limit it to chemical reactions, or grant them half the power of the Speed Force but not the "protects your body" part, so they rip themselves in two. Those would be problem powers.
The solutions or workarounds that I've heard of or used are:
- The power is off the table. This is the easiest for the GM to use. If the power+player combination is problematic, don't allow it. As Gerry Saracco said in the thread on the Supers! board, just because the power is in the book doesn't mean you have to allow it. This has the advantage that everyone sees it and everyone understands it. The disadvantages are that cool ideas from non-problem players can be dropped on the floor, or that you didn't know that the power was going to be a problem when you okayed the character.
- The power has limitations. This is the opposite tack and it can be applied retroactively. For some reason, the character can't do those game-breaking things. Steve Kenson has explicitly said this about Super-Speed in Mutants & Masterminds—the superfast character doesn't clean up the entire fight before anyone gets there because that's not the way things happen in the comics. It's a genre limitation.
- There are consequences to the action. Look, if the character casually scans minds, he or she is going to learn some unpleasant things about people, like what people really think of the PC or become the target of a shadowy conspiracy by learning something that was best kept hidden. The character is constantly blipping back in time to fix things? The timestream police come near and want to have a talk with the character...and they have specifically made themselves immune to his powers. (They've had eternity to research them.) Though I hate to use game mechanics for an in-game solution, you could take away a disadvantage (and its points) with the claim that it's entirely consistent with the character that he went back and fixed the problem in another timeline. (That last one seems kind of obnoxious to me, so I doubt I'd use it.)
Reasons could be the idea that you can go back in the past, you just can't be seen and change remembered history (he can timeshift, but he has to be careful not to be seen by the characters, so he puts the item they need in the potted plant...it's effectively a retcon, but the story is that he used his time powers), or it's very difficult to go in the past where you are, or the magic doesn't work that way.
Maybe the power only affects a certain mass, or volume, or the effects last only a turn (every power has a wreck-other's-fun rating that determines how long it lasts?) or it has side-effects that only show up a week later. It might not affect blue things, or green, or work in any room but the one where you did it previously.
If the player determines the reason, they're more likely to be happy with it.
On the bright side for the player, limitations might give the character more spotlight time. This is the Kryptonite rule: Whatever the character's weakness, it starts to show up much more often in the campaign. If your N-ray vision can't penetrate iridium, there's a lot more iridium in the world. Giving the GM a limitation is an invitation to create situations to your character.
As a GM, you have to be wary of the Kryptonite rule. And that leads to the last "workaround." Sometimes, the PC can just do it, because that's not what this adventure is about.
That's one of the big ones: If the character can do something, letting the character do it is fine, because the adventure is frequently about something else. If the character is super-strong, you never complain, "He lifted that car I put there and de-railed the entire adventure!" You put the car there to give him a chance to show off. If you need him not to move the car or rip the vault door open, then the car is attached to a bomb that will kill all the normals in the area if moved, or the vault door is made of Questonite, or something else.
As a GM, you're already used to thinking about when a power is important. My sense of consistency is such that I don't want a power that waffles in its abilities so I try to enforce consistency, but sometimes we hand-wave in order to get to the real story. ("Yeah, you can find rope in this barren warehouse and tie them up. Now the sigils on the floor tell you....")
EDIT: I've gone back to this several times to try and clean up various sentences. Nothing major, but I do struggle to make this clearer. Partly, of course, because it's a jumbled mess of thinking.
1. I think it's Robinson Crusoe where the hero swims naked out to the shipwreck and then stores useful items in his pockets for the swim back. Revision was less common in the eighteenth century.