I'd define something as being overpowered the moment a single item takes over and makes all "other ways of doing it" obsolete.
The problem is that mod authors each want their own mod to be useful so they keep upping one another. Minecraft is becoming easier and more automated with every new mod addition. The second one (automation) I'd consider a good thing but the startup cost and the knowledge how to set it up is sometimes too easy compared to the alternatives.
I don't mind Gregtech and Bee rewards for example because they take quite a lot of time, knowledge and space to set up. I also applaud Thaumcraft for its sense of balance while adding completely unique items as well.
Making the switch from SP to MP has changed my perspective on what is balanced and overpowered though.
I personally love mechanical systems, pipes, tubes, logic gates circuits, wiring, you name it. But lag and server friendlyness also play a role now.
Back in the days without teleportation, setting up a quarry was rather troublesome. Now it's just a matter of hooking up only 2 blocks next to it... Combined with chunkloaders and the server running 24/7 means getting stuff is many many times easier and faster compared to SP.
But because I have already done it the other (harder) way, I don't mind it becoming easier with 'magic' teleportation this time. Especially when it's a lot more server friendly.
I think a large part of the 'subjectiveness' discussion has to do with how many times you've already done the same setup before. Once you've "been there done that", having another improvement seems more interesting.
As far as personal pet peeves go; mine is spawners, especially with soul shards.
They remove any need to get items the way it was intended. I like the idea of going to the end when you need ender pearls, or having a reason to visit the nether when you need blaze. Having it all in infinite quantities right underneath your base just diminishes the need to ever go to those dimensions ever again.
They're also magic. They just spawn unlimited mobs (and thus items) out of thin air. At least with EE you have to input something.
And thirdly, worst of all, the sheer amount of items you can get with spawner farms also means mod authors keep making their recipes more and more expensive till the point where you almost *have* to set up a spawner.
Even being able to pick them up with a portal gun changes things and now with soul shards they have become so mainstream that it's hard to call them exploits though, even when they do break certain core gameplay mechanics imo.