I am not an expert in this, so please feel free to correct me:
A general guideline
A skill or the synergy between skills is out of balance in comparison to other skills in the game (in terms of dealing, boosting or countering damage).
Thus this is always relative and often topic of discussions within the community.
In my experience there are two possibilities in which a skill or skills can be referred to as overpowered:
1. A skill by itself is overpowered
This happens less often.
A typical sign (not proof) for this is if you find one skill is used in the majority of skill bars of one profession, allowing to deal or counter significant more damage then other skills of that profession.
PvE examples are:
"There is nothing to Fear!" and
Critical Agility- before the first nerfs -
or an actual example:
Ursan Blessing (which allows players to fulfill otherwise more difficult tasks in a much shorter time - see
Mallyx Thread).
2. Synergies (combination of skill and circumstances) between two or more skills make a build overpowered:
A sign (and again not proof) for this is if a
meta evolves for a certain profession including one skill that usually defines the rest of the skill bar as well.
[skill]Burst of Aggression[/skill] used by Assassins a while ago can be seen as an example.
This can also refer to a whole team:
An example was the so called "Rit spike builds" which allowed to deal and counter significant damage repeatedly in a short time over a long periode, making it hard to counter.
Concerning your question, "
why do they suddenly become overpowered after all this time?":
Well, sometimes an overpowered skill can be "sleeping in the shadows" till synergy is found.
This can be because other skills are introduced to the game or just simply because some bright mind found a great build after all this years (it takes some time to read the skill descriptions).
I hope this helps a bit to answer your tricky question... may others be more helpful then me
Cheers,
Timebandit
Edit: HAHAHA took me too long to write this ... tmakinen's answer include my points in a much shorter and more elaborate version = gg