When I think of overpowered I generally tie it to how many hoops you have to jump through. How many hoops do you have to jump through to make a mage/fighter/thief effective, how many more to make them powerful, how many more to make them insanely powerful. IMO most of the class types should not have to jump through many hoops to be effective, generally leveling up and acquiring loot should handle that. If the game is easy at that point, then I think you are overpowered.
But once you start having to jump through extra hoops I am not too concerned with being overpowered since you are making a conscious effort to gain extra power. If those hoops are too easy to jump through that may be an issue. But when people talk about the overpowered mages it is usually a guy who enchanted everything to boost his magic, who stacked weakness effects to have stupidly powerful spells and has a bucket of potions. That is quite a few hoops some easier than others, some are if it weren't for the lame level scaling I'd call an exploit but it is on the high end of hoop jumping IMO, especially considering how long it takes to level skills if you don't game the system on that. I am not sure I'd call that kind of overpowered a big problem.
This is a nice summing-up of the issue.
Yes - if a player just plays along and sort of falls into being overpowered, that's a problem that needs to be addressed. However, if a player invests a considerable amount of very specifically directed time and effort into the specific goal of becoming overpowered, and ends up overpowered, I see nothing at all wrong with that, and I don't see how the fact that others might take issue with that is even pertinent.
As you say, it's all about how many hoops one has to jump through. If one can only become overpowered through a conscious and deliberate round of hoop-jumping, I don't see the problem. Anybody who doesn't want to become that overpowered is certainly not being forced into jumping through all those hoops.