1. A low profile and firmer cushioning than typical road shoes: To run trails you need to be in touch with the ground, and on the softer surfaces of trails there is no need for over-cushioning.
2. Breathability: Trail runners by definition do distance and breathable fabric is very important for long term comfort. Always try before you buy by blowing through the toe box in the shop.
3. Good fit with no internal pressure points: Important in any shoe, but critical for long trail runs.
Features which depend on where you run:
1. Grip: Usually the first thing new trail runners think of, but most Australian trails are so easy they can be run in road shoes. Most common requirement is chunky tread that grips for downhill on dusty loose firetrails. Avoid closely-spaced treads which tend to retain mud and grass if you often run on these surfaces when wet. If you run often in mud, the pyramid studs of english fell shoes have proved their worth (eg Inov8). For wet rocks, soles with rubber give more grip.
2. Firm upper even when wet: Important for off-trails use. The classic orienteering shoes are a perfect example and will support you even while contouring round long slopes where normal shoes lead to blisters.
3. Light weight: While not essential, most trail runners appreciate not having the distraction of heavy shoes, especially when they get wet.
4. Drainability: If you cross lots of streams the water needs to be able to exit (you can always cut holes)
5. Waterproofing: Important in cold weather climates (but needs to retain breathability for runners)
6. Protective toebox and upper material: Improves durability immensely if you do a lot of off-trails running in the bush.
Things to avoid:
1. Wide heels: increases the risk of rolling your ankle on rough stuff. This sort of feature comes from the marketing department, not trail testing.
2. Too many ‘new’ features: getting the fundamentals right is more important than flashy ‘innovations’, eg lacing gadgets that don’t work.
3. Designed in gaiters: if you cross lots of streams you need something to keep small stones and mud out around the heel, but wearinglightweight gaiters works better than add-ons to the shoe.
4. Spending too much money. Most Australian trails are so easy and level they don’t need overbuilt and overpriced shoes.
Source: This article has also been posted on CoolRunning and you can see more discussion there: coolrunning.com.au/forums