they are made of a slicker material that bothers some people. it tends to make the bag slide around on one's back. the older ones were made of a more denim feeling material.
i actually prefer the plastic buckle to those rusty metal clasps on other bags that eat into the strap and make it fray. the buckles are plenty strong enough (believe me, i've carried literally tons in mine over the last twelve years) and they are easier to cinch than, say, a manhattan portage clasp.
having said all that, i have been really into the crumpler bag i won at MT6. so much lighter than any of my other bags. the crumpler isn't waterproof, but then "waterproof" bags aren't waterproof when it comes to messengering. you reach in so often that breathability is more important than a plastic lining that will keep in the wet. wise men rely upon a supply of plastic bags in their messenger bags for waterproofing.