Hmmm, well I have differant opinions on things from a lot of people.
When it comes to fighting games for example I think "overpowered" comes down to the amount of effort needed compared to the results you can acheive, rather than what the actual tier list is in the hands of masterful players who play on a high level and do everything perfectly. I tend to rate things based on when two fairly average players go at it. I tend to see see characters like Ryu, Ken, and Sagat as being overpowered even if they aren't strictly speaking top tier. The reason being is that moves like say Ryu's fireball are very easy to perform and very versatile, far more so than they should be for a very simple down, forward, punch manuver. While in absolute terms a fireball spammer or someone with a basic zoning strategy is easy to beat for a skilled player, for more average players that's game breaking because the power is far greater than the effort put into it. By extension I think this is the issue with characters like Wesker, Sentinel, and various Soul Calibur characters like say Mitsurugi which someone mentioned, it's not that they aren't quite beatable for a good player, it's just that the do far more with so less little effort. Your guy who "easily beats Wesker" for example is probably playing on a much higher level than 99% of the players who do fighting games will ever be at. People talk about going to tournaments here, and just by doing that your putting yourself on a level of skill that kind of disqualifies you from discussing such things as they apply to most of the fighting game population... your simply to good to be relevent to a discussion like this.
Otherwise, Game Balance is king and I think developers need to understand that if they want any kind of competitive or semi-competitive multiplayer title, they need to seriously put game balance above everything else. Something being fun or cool doesn't matter if it's not balanced for the rest of the game. This is what most of these problems are ultimatly causd by.
I'll also say that "overpowered" can be somewhat subjective, for example in the scope of something like "Dawn Of War" most people will tell you that the "Baneblade" is overpowered, but overall the Imperial Guard faction isn't. Having the most powerful (arguably) ultimate unit is part of their overall balance, given that as a faction they tend to be a bit underpowered toe to toe unless you can control the engagements perfectly with a lot of situational units. Overall the stronger a faction's general unit makeup (which might be a matter of production speed and replacement rather than individual power) the less powerful their final units happen to be.
In general for example Space Marines have perhaps the most powerful basic infantry unit in the game, a properly prepared tactical squad has a reasonable chance of going toe to toe with just about any unit in the game for example, and will probably hurt/weaken it even if it loses, in comparison their final unit that Land Raider thing tends to be one of the weaker "ultimate" units a faction can have.
In some kinds of games, like strategy games, the overall picture matters more than criticisms of any paticular unit. In that game series I tend to point to the Tau as being generally overpowered because of the way stealth works and how reasonably it easy to pick off or outrange stealth detector units. A stealthed unit against something that can't see it, can't even be targeted and this can be easily exploited. I suspect it wasn't tested enough in any of the games, and while it can be countered, it remains OPed because it takes far more effort to counter than it does to perform.