I think a lot of it depends on what level of slow-pitch you are working as well. For me, I think the most exciting is about the Class C and D level.
The classes above that are pretty much all power games and basically just sitting back and watching the guys hit bombs and whine about the zone. They are supposedly the better players around so you want to be on your game as well, but for me anyway, it's a much more boring game to umpire.
Below that and the players and pitching aren't very good which makes it a difficult game to umpire. You never know where they are going to throw the ball which makes if more difficult if you are working solo. It's a struggle to get any strikes called so you have a lot of hitters that stand there looking for a walk, which really irks me.
That C and D level, in this area anyway, seems to have the best balance for me. Decent pitching, not a bunch of power so there are more calls on the bases which get me into the game much more. They are generally experienced so you can do a bit of anticipation as to where the ball is going to go, which is a great help working solo. The pitchers are usually around the plate so there aren't many walks and you do get the occasional strike-out which is always fun.
The worst I ever had was a Women's Class F league. We are talking girls that have pretty much never thrown a ball before. One hour time-limits and I think I had only one game all year NOT called for the time-limit. But the scenery was very nice which made it a bit more bearable.
