I've never seen any evidence that .net's don't do as well as their .com counterparts, although I've heard this before.
Keywords in the domain will give you a slight boost but it's not the holy grail...that would be content. Ink particularly seems to like keywords in the domain but if you pay them they become blind anyway. Google also likes them, but again, the content behind the domain name is what really counts.
Think of how many sites there are that rank well without any keywords in the domain at all...much less hyphenated.
Bottom line: it helps a tad but don't bet the farm on a domain name alone getting good rankings.
Lets say that you should SEO a site for cottage rentanls And your main Kw´s its cottage rentals i think you will give a better boost for www.cottage-rentals.com thats my personal experience. But i agree with you there is alot of pages with no kw´s in url that have top listings thats why content is king.