The number one thing to keep in mind when choosing trail running shoes is how they fit. If they don't fit right, they're not going to feel right, and they're not going to work right. The best way to pick a shoe is to go to your local running store. They will recommend a trail running shoe based on the way you run on a treadmill in the store.
Things to look for: cushion and traction
You’ll need a shoe with a good amount of cushioning and traction for trail running. With a mid-level shoe, you can use it for trail, track, treadmill, and road running, and even racing.
Gotta have sole: rock plates and thickness
Elastic laces are great for off-road triathlons, because they allow you to slide in and out of the shoe very quickly.
Insoles give more support in the toe, heel, and arch areas.
For off-road racing, go with something that has high ankle coverage. You don’t want dirt, rocks, or sticks going down into your socks.