In crusader kings 2, what decides which titles I can revoke through plot?

The "Revoke the County of ___" plot only requires that a count level vassal hold more than one county (or that a duke holds a county outside of his duchy) and that you not be incapable or an imbecile. BUT meeting those requirements does not ensure that the plot will show up. There is also an apparently random element in which plots appear and that random element is affected by a number of personality traits and other factors. So a "Content" character is very unlikely to see this plot. And a "Kind" or "Slow" character is somewhat unlikely to see it. But an "Ambitious," "Decietful," "Envious" character who meets the basic requirements will almost certainly see it. The chances of it appearing are also affected by your opinion of the target. If you hate them (according to the game's calculation of your opinion) then it is more likely that you will have this plot available.

At least that is how I read the files. I'd welcome anyone who has more insight into this.

You need to have a strong claim on the county and be the liege lord of the count. Then you can hatch a plot to revoke it.

But if you have strong claim and low (or no) +crown authority, you can revoke the title without tyranny penalty through the diplomacy menu.