Erik Darling has a short checklist of some things to check before upgrading SQL Server 2005:
MIND YOUR COMPATIBILITY LEVEL
When going to 2014 (as of today, 2016’s RTM hasn’t been announced yet), you’ll have to decide whether or not the new cardinality estimator suits you. There’s not a cut and dry answer, you’ll have to test it on your workload. If you’d like some of the more modern SQL features added to your arsenal, you can bump yourself up to 2012-levels to get the majority of them.
The interesting survey would be, among people who still have SQL 2005 installations, how many will move as a result of Microsoft declaring end-of-life for 2005. My expectation is a fairly low percentage—by this point, I figure a at least a strong minority of 2005 instances are around for regulatory or compliance reasons (e.g., some piece of regulated software was certified only for 2005).