True, many are worthless but of those that are not they simply want too much money. All too often you see a ridiculous price and just accept that domain is never going to be used for anything.

I made an offer on a domain once and the immediate response was 'I already have been offered more than this'. Obviously he thought this was some great negotiating technique?

As it happened, I didn't increase my offer and, a few months later, the domain dropped and I got it for the basic registration fee.

I have had a few good experiences though where the person holding the domain was professional and would accept a reasonable offer but, anecdotally, they are a minority.