I recently competed in a trail run - a rainy, nighttime trail run, in fact.
It was in a mountain bike park, so involved a 19 minute hill climb, 4.5 minute ridgeline run, then another 19 minute stretch that was an overall descent but undulated a lot.
I've been a runner for nearly 20 years, completing a handful of half marathons (never quite having the stamina to do a full one), but I don't do much road running (beep beep!) now, preferring to go off-road.
The differences between trail running and pavement pounding are both mental and physical. Yes, it's usually hillier and slippery, but the real difference is the concentration required. In road running, you can get into a rhythm and let your feet carry you along, only needing to think about the occasional curb or to look out for traffic.
When trail running, the perspective is totally different.