Features to consider:
Double butt; double knee
The heavy wear areas.
Full-side zips
Two reasons:
1. It's possible to put the pants on w/o removing boots (to-the-knee zips also allow this, but not as well)
2. Ventilation
Low profile side zip
Makes the pants a lot less bulky. The zips on the Arc'Teryx products (for example) are first rate. After that internal storm flaps seem to be less bulky than external storm flaps.
Beefy crampon/ski-edge patch
on the inside ankle. In the absence of a rugged external gaiter, such OR Crocs, the benefit of this feature will become very apparent, very quickly.
Removable suspenders.
Probably a personal preference, but they're nice to have on occasion, if not always.
Map pocket on the thigh.
Probably a personal preference.
Internal Gaiter.
Personally I don't like these, but some friends LOVE them.
As to the question about which products. Hmmm....
More than any other piece of gear (other than gaiters), these things seem to get trashed in normal use; glissading, errant crampon placements, ski edges, ice screw teeth, bushwhacking.... all take their toll on shell pants. Consequently I tend to think of them almost as a "consumable" and shop as much on price as anything else.
Gortex is great, but somewhat superfluous when it's possible to vent using full size zips. Pants made of other breatheable (and not AS breatheable) fabrics sold by Lowe Alpine, Patagonia, etc. work acceptably well and are often a lot less expensive. I've used products from Lowe Alpine, Marmot, Mountain Hardwear, and housebrand stuff from EMS; they're all fine - usually just a variation or two in the features that are offered - and worthy of consideration.
The online overstock / closeout places usually have some inventory:
www.rei-outlet.com
www.northernmountain.com
www.sierratradingpost.com
www.backcountryoutlet.com
www.campmor.com
And of course, good deals can be found on the close-out racks at bricks-and-mortar stores: EMS, IME, Ragged Mountain, and The Mountaineer, etc.