My tip number 0: Use a CMS. It's a beautiful thing to start an ec2 instance, run Ansible, get a drink of water, and return to a fully provisioned and configured machine. Tip 0.1: Use Vagrant with an EC2 box, and just 'vagrant up' yourself a fully operational machine.
Why are people so thrilled about a slower more error prone way to do what amazon provides already? Just make an ami for each kind of server you want. Now you skip the waiting part, and the "oops, my pile of messy ruby scripts fucked up configuring the server" parts.
You can use Ansible to create an AMI to get the best of both worlds - fast provisioning, plus the ability to make new AMIs in a repeatable, self-documenting manner. You're arguing configuration management is "error prone", but I've found it a lot less error prone than other methods. If something doesn't install right I can blow away the whole thing rather than trying to un-break a destroyed install.
If your Ansible scripts are in Ruby you've done something very odd, and if they're messy it's your own fault. Mine are pretty clean, and once built they work the same way for subsequent runs.