"what features found on a mountain bike (not found on a hybrid) would benefit my physical build. i'm 6'6", 255 lbs."
Another viable option is a bike designed for a Clydesdale (over 6 foot and over 200 pounds). Gary Fisher makes a series of mountain bikes called Two-Niners (mounted tire size equals 29 inches). I mention him because it is the first US production company to have that size bike in production (this being the second year of his Two-Niner series). These are mountain bikes that use 700c rims, but run the nice fat tires for mountain bikes (knobbies, semi-slicks and slicks available). I would assume that you would take the XL frame size and be able to dial in the fit perfectly with the proper stem and handlebar for your riding position preferences.
Other companies make 29" mountain bikes as well. Surly makes a nice chromoly frame called the Karate Monkey (comes with rigid fork) that you can build up as a hybrid, a mountain bike with gears, a single speeed and it won't set you back too much in price. You can view it here:
http://www.surlybikes.com/frames/karatemonkey.htm
If doing a build such as this (or another custom frame maker) - choosing sturdy hubs and rims would be a must for your weight.
Going back to a 26" wheeled mountain bike and features that might be good for your size would be the ability to choose rims, hubs and a lacing pattern that you would not be able to destruct. There are also a plethora of suspension forks that are targeted at your weight and size. A lot of the components used in Freeride and Downhill mountain biking are used for Clydesdales to insure a durable bike that can handle the load.
BB