Not a bad list micah. I'd probably add a few things like SMS rates and MMS ability (Twilio wins there too).
Couple of the biggest gripes I had working with Tropo (which I did quite a bit of before) is that their logs are awful to deal with which goes to account management. Last time I checked they're still FTP and you need to parse them. And probably the biggest one is the lack of knowing what happened to the call end. Twilio sends a POST at the end to a URL while Tropo does nothing for you and if it chokes for some reason you're SOL. To compensate for this we added a special "endcall" that each application was sent to to get a rough idea of when the calls were done. They do have an API now to check on the status but then you have to write some kind of cron thing and that's just ugly.
It's also worthy of note that Tropo has two kinds of APIs. Single page (which is pretty straight forward) and the WebAPI which acts a bit more like tradition HTML forms, do something, POST, do something, POST, etc.
We worked closely with Tropo early on at one of my old jobs and they're pretty good (backed by Voxeo) and they also have a telephony client with Phono that you can leverage which is pretty cool too. If they did a bit more on account management like Twilio, added MMS, and dropped their prices a bit I think they'd be back in the game. They don't really do as much marketing though like Twilio.
They used be a lot closer in the past but Twilio has really been running away with this one IMO. If they ever change their infrastructure to support voice recognition they could put this one away but that would be tough I think since that's not what they've built it for.