No more chainguide from Sram?
SRAM XX1 is their new single chain ring setup. Very expensive and I have not even tried a demo, very wide range on the rear 11 speed cassette (10 x 42) and lots of chainrings available for the front.

My understanding is that the chainrings are designed to 'hold' the chain on. The chainring teeth are designed (wider and taller) to have a better grab of the chain.

Good review/article here: http://www.bikemag.com/gear/first-impressions-sram-xx1/
Good image of the chainring enlarged teeth here: http://www.bikemag.com/gear/srams-1x11-group-new-details-emerge/