One of my favorite games currently is Dungeon Fighter Online. Its a F2P MMO for PC that has one major restriction: a fatigue bar. Basically, it means you can only run so many dungeons per day. While normally this would be a huge negative, in a more modern environment that eats up so much of my time, knowing I can only put so much work into a character at once is actually incredibly freeing. My dailies take about half of the bar at max level, which still gives me plenty of time to run other stuff if I'm working on something specific. While I understand that can be a major barrier of entry, I enjoy it quite a bit, because it makes me less pressured to spend extra time I dont have trying to level a character.