In a vocal thing or piano, the melody should be strong with a good bridge/chorus that keeps me interested. A fingerstyle piece can have a strong melody or a good "hook" that is repeated with variation and keep me interested. I don't mind instrumentals that are "guitar centric" to quote a certain AGF member
as long as the "hook" is strong and repeats enough to tie it all together.
I don't like instrumentals that seem to be endless and without a central melodic theme that repeats, even if the guitar does sound great.