They're not particularly prolific. Most of them are kept within small circles for quite a while, not known to the public until either after being fixed, or immediately before. (The latter leading people to believe the only way to get game companies to address them is to spread them around so they have no choice but to act on them.)
There have been numerous minor skill bugs that led to exploitation during the time period where they were broken, in addition to a few more serious griefable ones, such as the team PK bug.
One of the most annoying skill bugs, in my opinion, was the "Conditional endings don't work" one, wherein Vengeance didn't kill the resurrected ally, Battle Rage didn't end when you used a non-attack skill, and most impacting, Thunderclap didn't end when you ran out of energy.
The Vengeance one has been broken multiple times, and it often took a while to fix, but the bug where all of them were broken at the same time was taken care of in a fairly short time period.
The worst/best PvE one that I can remember is probably the Doppleganger one, actually leading to a number of the first post-release announced mass bans. People were able to kill the Doppleganger again and again for the experience bonuses by quitting during the cutscene and keeping them while being able to do it again... or something like that. I can't for the life of me remember the details, and it's fixed since then.
There are still plenty of lesser, mostly unexploitable bugs around that haven't been fixed. They tend to be on a rather small scale, such as a particular monster group not doing anything, or a skill description error, and you can report them all you want, but while I'm sure they're slated to be fixed eventually, they're just not high-priority.
The biggest thing in MMORPG's, though, tends to be botting. Bots, done right, can be set to emulate approximate human behaviour so well, particularly where humans can perform the same repetitive tasks easily, that it ranges from very difficult to detect, to impossible. Well, aside from a Turing test, but even that's manageable to a point.