"It's expensive and requires special equipment." In reality, testing costs can be fit into almost every site development budget, and the only equipment required is a computer, two chairs, and a notepad.

"Where will we find all the people we need?" Properly done, you can get extremely useful results from testing no more than five subjects, one at a time (unlike a focus group where people react off each other, usability testing is one-on-one).

"We need to find subjects exactly like our actual site visitors." Since site visitors are anonymous, you can never know exactly what they are like. Almost anyone who isn't involved in your web development project can be a valuable usability testing subject.

"I don't need to test, I have a lot of web experience and know the things to avoid." In fact, the closer you are to the development project, the more likely you are going to miss the obvious flaws in your site design. Have you ever tried to proofread your own writing? You always miss some "obvious" typos.

"Let's just test a bunch of people right before site launch and fix everything then." That's a sure way to blow your launch deadline. Testing is an iterative process, and should be done early and often.