I think the most "reasonable" definition of overpowered would be characters with inherent, overwhelming advantages.
For instance, any Sharingan user who awakens the Third Tomoe is automatically an A-rank ninja, and can probably fight evenly with the likes of Asuma / Temari / Sakura, regardless of other training.
The same would be true for any Jinchuuriki who can activate up to four tails worth of chakra; Naruto mortally wounded Jiraiya in a rampage. While it's obvious Jiraiya would not fight to harm Naruto, the fact that he couldn't avoid damage still speaks volumes.
I would not consider a Shinobi to be "OP" simply because that Shinobi is more powerful than average. "OP" implies some level of unfairness in competition, including the competition to increase one's power. For instance, Sasuke can copy Jutsu with his Sharingan; that is OP. The ability itself is OP, regardless of the technique copied; If Kakashi created Chidori, a non-OP move, and Sasuke copied it, Sasuke has just demonstrated OP-ness.
The same is true of Naruto using Kyuubi chakra to train for longer periods of time, or even Naruto's ability to use Sage Mode thanks to his abnormally strong body (Uzumaki genes at work). The latter is somewhat debatable, but at the very least his Kyuubi chakra is OP.
There are abilities which can keep up with these Uchiha / Jinchuuriki gifts, but the key is how the abilities are developed. If Orochimaru spends years developing / mastering Edo Tensei, I would not call him "OP", because there was nothing "unfair" about the way Orochimaru obtained this power; Naruto, Sasuke, or Tenten could have done likewise if they were motivated (and Sasuke would copy the technique with his Sharingan rather than train for it).
I would apply the same reasoning to Guy's Gates; they are a product of hard work and some talent, but not "OP" traits. Naruto, Sasuke, or Tenten could likewise have mastered the gates (and here, Naruto's enhanced stamina and recovery thanks to being an Uzumaki/Jinchuuriki/Sage would probably let him withstand the Gates at a higher level than Guy, and master them more quickly).