I have a North Face Cat's Meow sleeping bag that seems to have a locking system on the zipper designed to prevent it from being unzipped accidentally. Great feature for keeping you from getting cold, yet it seems to be the biggest annoyance ever when trying to use the bag normally. It's an odd multi-piece design that wants to be slightly closed (as if zipping up the bag) before it will release and let you unzip it. Great theory that is horrible in practice.

On the other hand, my Western Mountaineering bag (Alpinlite) has built in rigid area's on either side of the zipper that prevent any of the fabric from falling into it and getting caught, yet they easily overlay when zipped to prevent cold spaces. It also uses a regular (non-locking) zipper that works very well. That one's an awesome design!

Of course, that's just one difference between the $150 bag and the $555 bag...