I feel like I'm beating a dead horse here, but Dungeon Crawl Classics RPG is the most surprising game I have both run and played. Some people say, "It's swingy, and can't hold up to a long campaign." I have found that the mechanics are surprisingly stable. Yes, TPKs and character deaths can happen, but I have had no problems running a regular campaign.
I don't run your standard campaign, however. There is no big story arch with a fully detailed setting. It's more of an extended hex crawl, with only key places and NPCs being mapped out ahead of time. We build the world as we go and let the story build organically from the results of the sessions.