Run silent, run deep. Shimano's ZEE rear derailleur uses a short cage and a Shadow profile to keep tucked out of the way of rocks, roots, whatever. The Plus part is a one-way clutch system that stabilizes the cage and your chain when ripping over rough terrain. Chain slap and the noise that goes with it, as well as dropped chains, are virtually eliminated. Of course, it still has the smooth action and sniper-like shifting that Shimano is rightly famous for. Freeriders and trail riders can run a typical MTB cassette like an 11-32, 11-34, or 11-36. And DHers can choose the close-ratio version and run a road cassette like 11-23, 11-25 or 11-28.
An important part of a rear derailleur's job is keeping the chain taut as you change gears. Chain-wrap capacity is a number that tells whether or not a derailleur will be able to do this on the drivetrain you plan to use it on. Calculate chain-wrap capacity with this formula: Chain-wrap capacity = (large chainring - small chainring) + (large rear cog - small rear cog). Example: 44/29 chainrings and a 11-28 cassette: (44 - 29 = 15) + (28 - 11 = 17) = 32-tooth chain-wrap capacity.