With all the buzz around the possibility of an 'RDF-Lite', I feel compelled to list a few barriers that I think URIs raise for a new user trying to get to grips with RDF metadata creation.
Here they are, in no particular order:
(1) URIs don't allow you to use existing identity schemes.
Apart from existing web resources, URIs currently aren't used in everyday life which means that a new URI must be created, described, maintained and promoted for each resource you want to describe in RDF.
(2) HTTP URIs have a load of implicit baggage
Hash vs Slash, how to distinguish web and physical resources, how to keep URIs persistent. These must be understood and up-front decisions made with important ramifications down the line.
(3) URIs are URLs
This confuses everybody I try to describe RDF to. Regardless of philosophical debate, the most commonly accepted use of a URL is to denote the address of a web resource. Using them to denote real-world things confuses people and (as a side effect) creates messy debates about how to denote existing web resources, how to describe resources on the web etc...
(4) URIs require a level of precision in 'meaning' that is hard to attain.
URIs are globally scoped, which means they need to mean the same thing in any context. Whether this is actually possible on a large scale is a subject for debate, but regardless - using URIs collaboratively and successfully requires a non-trivial amount of upfront thought, documentation and proactive consensus building.
So there you go. Any others?