It really depends on how much you let it hurt you. For example. I have few domains with dashes, but they are pretty great domain names, so I don't worry that much: orthodontic-braces.com (the main keywords for my niche), new-york-doctors.com - something I want to develop in the next months to get more into the medical niche etc.
I can't imagine having a - in my .com domain. It is harder for people to type and remember. People are generally lazy, that is why 4,5, and even 6 letter domains go for so much. If I am serious about my website I will not waste my time with a -.
For many people, this is obviously a subjective subject. As dojo said, I think it comes down to knowing how to use it to your advantage. I would personally avoid it, if possible, and keep it short and sweet, but if you absolutely must use a hyphen, there are ways to make the most of it.
Ideally, it would be one word, but realistically, you don't get what you want as there's always some squatter out there snatching things you want.
Aesthetically, a dash feels a little more natural to me than just concatenating two words, and as for non-.coms... They not only look unprofessional, but people find it really hard to remember alt domains to a terrifying degree.
How i see it. At first - names might be hard but after awhile it might work in favour as its i guess more unique. Like forumpromotion. Oh wait i wanted forum-promotion as its the - that changes it.
Then you might get people who forget it aswell.
Depends on the wording used to seperate the dashes