The main problem is too much of an obsession with words and not enough attention to sense. I prefer professionals who are prone to talk less and say what really counts, idiomatically, with elegance, precision, natural intonation and poise. I find it difficult to put up with practitioners who sound bored and boring, or have a halting delivery, or scarcely pause to take breath and then at the wrong places. In that, I am irritated by the same things that irk any listener in any speaker. I want my interpreters to be top-notch communicators.
Versatility is also important.
I try to avoid relay at all costs. Other things being equal, the first interpreters to get the offers are those with more relevant passive languages. Colleagues equally at ease in two booths come next. Professionalism involves several series of factors, having to do respectively with the interpreters' attitude towards his audience, his colleagues and me. A decisive factor is, of course, thorough preparation for a meeting.