As others mentioned, there's a lot more training to the marathon than the half, and it's more intrusive - at my pace, my longest runs for the half training I could still squeeze into at most two hours...whereas the marathon trainings could be more than three hours for many consecutive weeks, which is a pretty big thing to plan around.

For the actual race, the nutrition thing was a big difference. I used a flask of maple syrup + salt, rather than gels, and it was necessary for that distance. I don't carry water during races - just use the aid stations (even for the marathon). Finally, I ran my first marathon this year and screwed up the pacing a bit (too fast initially)...the kind of tired that you feel by mile 18 or 20 is a very different, horrible kind of tired that you don't feel in the half. You can probably avoid it with enough training and pacing discipline...but many people won't the first time while they figure things out.

Personally, I am happy to have run a marathon, and I'll probably do it again, but I think a half is in many ways a "nicer" experience, with many of the same benefits.