From the mouth of the project's owner:

From skimming the docs, it looks like Esper is much bigger, much cooler, and with a more abstract version of events. It implements a lot of the primitives I've been considering but haven't built yet. It looks more difficult to set up, and has a commercial offering for support and HA; neither of which are present in Riemann right now.[1]

I recently finished building a monitoring system using Esper and JRuby since the client asked specifically for that, but I wished I had used Riemann from the beginning.

[1] https://groups.google.com/forum/#!msg/riemann-users/GhVMYJow...