• Low profile
• High traction soles this includes specialized "sticky" rubber for rock running and scrambling. For example: the top section of "Old Rag."
• Stable, wider base
• Toe box protection (this trend is declining, opting for lighter shoes similar to traditional road running shoes, but should be considered if you expect exceptionally rugged terrain, e.g. Hard Rock 100, adventure racing, etc...
• Increased mid-sole cushioning for running over rocky terrain
• Many have Gore Tex construction for extreme weather conditions (this can lead to over heating during the summer).  This is a great feature for sustained snow running (especially wet snow).
• Quick drying--this may be at odds with the feature above
• Summer trail or adventure racing--look for mesh with highly breathable and the ability to drain water for course that feature river crossings and/or swamps
• Shoe weight--If you are primarily competing in shorter races consider a light weight shoe similar to the La Sportiva Slingshot. Many of the 2008 trail shoe models are lighter than their predecessor.
• If you are racing in ultras consider more cushioning, support and stability for the long haul.  This also depends on how how light on your feet you be at mile 75--some runners will still want the lighter shoe with more feel for the trail.

Go here for women specific trail running shoes>>

Running shoe manufacturers recognize the market potential of trail running shoes and there are now countless styles from both traditional running shoe product companies and climbing shoe manufacturers as well as traditional hiking boot product companies which leads to confusion for the first time running trail shoe buyer.  Trail running shoes run the gamut from light hikers to racing flats with very sticky outer sole and little lateral support.

After considering the trail running shoe characteristics listed above, match the trail running shoe to the most likely set of conditions.  If your primary trail running use consists of "rails-to-trails" terrain with reasonably smooth and even surface then you might consider a lighter weight trail running model or even a traditional road racing shoe.  Having just completed the Deadwood Trail Marathon, most runners did not have traditional trail running shoes, but opted for road running footwear.  Given the current vast number of shoes on the market, I recommend at least two different shoes: A light model for the summer and a more weather resistant model for the winter months.  Consider Gore Tex shoes if you are using the trail running shoes for snowshoe racing also.  Even in the coldest conditions the warmth of your feet melt some of the accumulated snow and over several hours will lead to wet feet.  Also, seriously consider using low cut gaiter in snow with or without the snowshoes.

Winter trail running tip :  Use short hex shaped head machine screws to provide traction on ice and packed snow. Screw the hex screws into the sole of the running shoe (the hex head provides the traction not the point) being careful not to damage air bladders.  More on this technique from Matt Carpenter's Sky Runner's site.  I used 1/4 inch screws last year and had absolutely excellent traction on ice and packed snow.  This technique does not provide a great deal of traction in deep powder because there is nothing for the head of the screw to bite into.

Bottom Line: Select a trail running shoe to fit the type of terrain (water crossings?), race distance, and weather conditions.  I have used a couple different Salomon shoes for adventure racing including the XA series.  I have used the La Sportiva Slingshot for shorter trail races and interval training sessions and the Axis Eagle V and Trail Sensor for the Leadville 100 Trail Races.