Here are several adjectives that I have heard used to describe proofs:
Wrong, false, flawed: there are serious problems with the logic used.
Incomplete, weak, partial: fails to completely address the statement being proved.
Hand wavy, outline, sketch, reductive, imprecise: the proof is heuristic, giving ideas and motivations without being rigorous.
Clunky, confusing: the proof is correct and rigorous, but poorly structured.
Terse: the proof is so short so as to be confusing.
Wordy, long winded: the proof is longer or more complicated than it needs to be.
Heavy-handed, overpowered: the proof uses techniques or results which are far more powerful or advanced than need be used.
Inelegant: the proof uses simple but time consuming techniques, when better ways exist.
Technical: the proof uses long, possibly boring arguments for which there is no reasonable alternative.
There are positive ones too.
Clear: the proof is very easy to understand.
Concise: the proof is short by eliminating unnecessary phraseology.
Elementary: the proof does not use advanced techniques, especially when more advanced proofs exist.
Clever, interesting: the proof uses ideas that are are not obvious, or may even be novel.
Beautiful: a proof that is very clever, clear, and well written so that it elicits an positive emotional response.
Cute: a simple, clever proof that is not quite beautiful.
Elegant: the highest praise a proof can receive, such a proof is beautiful, and relies on a simple but not obvious idea.