So much has been covered already.  Trying not to repeat, I add the following:

1) Clients that refer you to client's like them (more good clients)

2) Trust - Clients that ask for services without shopping around for 6 quotes and then beating you with them. I have this one client that I love. He just makes his requests and pays his bills. Never asks for a quote. But, he's been a client for 4+ years. That's the gift that keeps on giving.

3) Projects where everyone clearly sees a difference and correctly attributes it to the project. Celebrations are fine too. ;-)

4) Talk and Listen. It's awesome when a client knows what they want. Also aweseom when they can be taught that they have in backwards and proceed accordingly.

5) Clients that don't refuse a proposal, but instead tell you what they can do and requests an appropriat revision of the proposal, that will be signed.