The bottom line here, I think, is consistency. People expect things that look similar to work the same.
Jakob's Law of the Internet User Experience: Users spend most of their time on websites other than yours. Thus a big part of customers' mental models of your site will be influenced by information gleaned from other sites.There's a great example of the confusion people have between search boxes and browser address bars.
I've seen a classic example on the University website lately while analysing search terms. Our search box has some text in it by default - "Search term" - which the user is expected to delete or type over before searching. Guess what he number 1 term searched for on the University website? Yep - it's 'Search term'. Thousands of people every month click the 'Go' button without entering what it is they're looking for in the search box.
Mental Models - article by Jakob Nielsen