Reminders App Hangs after Mavericks Upgrade
I started to do the same as mentioned above -removing all location-based reminders including completed ones... (had to go directly to iCloud.com)

As I was going through, I found one that had "location: null", deleted it (or just removed the location part of it) and then, before deleting any more, tried my reminder app on my Mac computer (running Mavericks that wouldn't open/crashed). AND IT WORKED

So to be clear, you don't have to delete ALL location based reminders -just any that happen to have "null" in the location which, of course, messes with it's head! (I must've bumped it on/not done it right).