I've totally converted to trail running. If my knees wouldn't suffer, I'd go out every day.
It's so much more demanding than road running. You just can't check out and plan your meals or practice that difficult conversation you want to have with your boss while trail running as you can while on the road.
You have to watch every rock and root and plan where you're gonna plant that left foot, right foot ... and duck those branches and plan how to avoid continuous braking on the downhills (don't do it!!! knees won't be happy!!!), and instead think about cushioning those footsteps on the downhills (well, I do because my knees demand it), and choose which rocks for lunge off of to get more speed, and figure out which way to go in the mud, and so on and so on. Very entertaining! Oh, and it's especially fun when you get lost, or when you're first figuring out where a trail leads.
Anyone on the north shore of Boston out there? I'm learning all the trails around here, one at a time. Have got a bunch of them down by now. I can point you to the hemlock stands, the loose gravel and ledges, the BC-style elements that have been built (the terrain around here is quite gnarly, but some people just have to go overboard), the flats and the hilly sections, the muddy and the dry trails. I'd love company. I run slow!