I'm thinking about transferring some sites from godaddy to hostgator, and I'm wondering how scared I should be? (this'll be my first transfer) what can i expect in terms of rankings?
Just make sure that you have you site fully transfered to your new host before you change over the nameservers on your domain and you should not have any issues with your SERP.
You could also set you TTL (time to live) to like 300seconds on your domain a few weeks before the xfer and when you do it should be really quick.
Just have exactly the same website on both web hosting accounts during the domain transfer process (which takes up to 48 hrs) and then when it fluctuates between both hosts, you should be fine with your site still online. This way it shouldn't hurt.