You have a good point that user_name validation is the same whether or not you use a DB or not, however, if you have been leveraging an RDB for doing validation, queries and collection management, then making the transition to doing all of your work in objects (i.e., using an ODB), means that you need to start thinking in terms of objects everywhere.
This turns out to be a fairly common question from folks transitioning from RDBs
GLASS is aimed at making multi-vm/remote-vm development easier.
GLASS uses continuations for debugging stacks from remote servers...when an error is encountered, a continuation is snapped off and stashed in an ObjectLog (a persistent collection). At the developer's leisure, she can bring up a debugger on the error stack in her development vm.
If you are debugging in a development environment you can proceed from the 'remote continuation' and the newly rendered page will show up in the browser...