1. Reputation: We would stongly suggest to look for a reputed shared web host. Someone who is in the indutry for few years and have a few hundread thousand sites hosted is going to be a better bet than a web host started yesterday. Checkout the web hosting reviews and hosting forums.
2. Support: Look for someone who responds to the supports tickets fast and is available 24/7 on toll free lines in case of an emergency. Some cheap shared web hosts are known to have a poor customer support.
3. Resources: Now a days, every cheap shared web host oversell. So you are most likely to get more than what you need.
4. Price: Shared web hosting has become cheap over the years. Why pay a premium when you can get it cheap?
5. Features: Do they offer cPanel? Then you must get most of what you need. Still checkout the web host provide all the features you need: mysql databases, .htaccess, add on domains, script installer, ruby on rails and so on.