In addition to the things you'd normally look for in a host, if you're going to be running a Rails app go with a company that offers:
- Shell/SSH Access
If you can't get to the command line don't even think about it. So no goDaddy, period.
- Mod_Rails (Passenger) and/or Mongrel Support
Rails is way too slow and unstable on cgi, fcgi, etc...
- The Latest Ruby and Rails Releases
If they don't update their server fleet quickly it can be a serious security issue.
- Support Staff Who Know Rails
Too many companies respond with something like 'that is a scripting question and we're not paid to answer that' — which is lame.
- Rails Documentation
If you email support with a question like: 'Help I got a FastCGI: (dynamic) server ... has failed to remain running for 30 seconds error in my apache logs...'
and they don't point you to some documentation they have already on the issue. You may want to find another host.
- Scalability Options
Do they have plans that meet your future needs if your app gets big? Will they setup a good Rails
stack for you on a VPS or dedicated server (even if they don't advertise it)?
Will they help you into a multi-server Rails environment if necessary?
Can they move your account between boxes/plans seamlessly?
- Pre-installed Gems
Its nice to be able to run 'gem unpack BlueCloth' or something and have it already installed on the server.
- mySQL, PostgreSQL, and SQLite Databases
It is extremely helpful and nice to have these options if you're developing apps that soar on (or are designed for) one or the other.
- Git, Subversion, Trac, and/or Buzilla Hosting.
Not only are these helpful apps, but its a good sign that the company supports developers.
- A Website or Applications of Their Own Developed with Rails!
This is certainly a good sign that they are serious about the framework and its use in a production environment.