A lot of blogs and CMSes have lengthy keyword stuffed URLs and seem to do okay. But you do run the risk of reaching the cutoff point for URLs and having indexing problems or lost keywords. Try to keep it short by removing words with less then four letters.
descriptive urls are beneficial for more than just SEO... it's quite common to use PHP or otherwise to serve up pages by an ID... personally, I don't like that.
methinks a good folder hierachy is something to aim for... if each folder has a short, meaningful name, it means you can give someone an URL to a page deep in your hierachy that describes itself.
A bit of subdomain-ing and request-level URL rewriting doesn't go too badly either... as long as you do it consistantly...
My site has a number of forums, each one's on it's own subdomain, each group is a folder, each board is a folder of each group, each topic is a folder of each board, (and in actual fact each reply is a folder in each topic, but that's never seen)... that kind of organisation is necessary for my processing system; and each folder is named automatically... so for example:
means "in the home forum, in the tos (terms of service) board of the help group... is a topic about advertising."
medon't think that's too much information in an URL, and medont think URLs are that important to search engines... the word "Google" didn't mean anything, (especially not anything about search engines); until someone put it in the oxford dictionary.... But nonetheless, Google comes in at number 2 (of unpaid links) in a Google search for "search engine"...