I don't mind running in semi-sketchy situations.  I don't mind running on trails that may or may not be closed due to bear orgys.  I don't mind skirting 30 yards around a buffalo cause he's looking frisky.  But I do mind running through thermal areas.  A large hunk of Yellowstone is in the caldera of one of the worlds largest volcanoes, so naturally one runs into areas that can get heated.  On the Ribbon Lake trail, after gracefully dancing with a buffalo, I came across a large area of white-grayish ground/rocks that had the smell of sulfur that reminds you of really old devil'd eggs.  Deciding against trudging across hollow, potentially break-through-and-lose-your-skin, ground I 180'd early.  The good thing about running in Yellowstone is that more often than not, more trailheads are nearby.  I luckily got the chance to go from Ribbon Lake Trail to the Lower Rim Trail and it was definitely worth it.  Within minutes I got the chance to run with buffaloes and big ol' buck elk through a large valley, then traversed into the Grand Canyon of Yellowstone skipping along with waterfalls and a ridiculously fast moving river. Who says being careful can't still be fun.