Another major factor for grads or advanced undergrads is the research that the professor is doing. This primarily comes into play during office hours (which are generally empty except before exams). Especially for established figures, this can be the only chance you'll have to get to know them (most are far too busy to take casual callers).

Even the worst lecturers are sometimes extraordinary one-on-one and, even when not, people doing interesting work tend to have far more than average contacts with other people doing interesting work. Show them that you're engaged and curious and invisible doors will open to you (many of the most interesting positions are never posted, but instead filled by qualified recommendations).