Plenty. Where to begin...?
To actually go into depth about it would take about an hour of writing, since a well-thought-out VIllain panel at a convention eats up at least that much through talk and discussion.
The quick and dirty of it is that you need to have at least SOME...style, wit, motivation (if even insanity), cool ability (if even mortal ability expressed well), and competence.
Let me take a very human example from the old animated series of Batman: THE CLOCK KING. Temple Fuget is an aged obsessive-compulsive man who worked at a successful firm ruined by one day when the later-to-be Mayor of Gotham told him to take his afternoon break fifteen minutes later than usual...and then terrible things befell him. How much it was the Mayor's fault, one cannot say. Perhaps his court date would've been settled properly if he hadn't stepped out of his inflexible range for a moment or perhaps not. We don't know.
Fast forward to years later, a man is sabotaging the Mayor's career and then ultimately plotting to kill him, foiling and fighting against BATMAN with nothing more than a few weapons, stuff that anybody can get off the counters or learn anywhere, and his OCD. The Clock King has a charm for being almost justifiable in his outrage and exceptionally cool in how he operates. He doesn't even look flashy, so it's easy to lose him in a crowd.
I think that's a good summary of a good villain.