I think what you're seeing is actually part of the design of the chainring. As part of their strategy toward smoother shifting, Shimano designed some of the teeth to be considerably shorter in key areas to aid chain deraillment and pick-up. Shimano has a marketing term for it, I can't recall what it's called right now. I had a friend who bought a new Cannondale MTB a while back and was sure he had a defective crankset when he first noticed the shorter teeth. It would be very difficult to have a couple broken teeth in a certain spot without some other noticeable damage. Bashing about on rocks might be about the only way. Likely there's nothing wrong with your chainrings at all. I agree they look odd, but it seems to work. Shimano's been doing it for years. Any chain slipping is more than likely an adjustment issue.