I haven't noticed much difference from the developer perspective. The really nice thing about the AST parser is how easy it is to add native functions to your chosen Dialect subclass using the SQLFunction interface.
---<br />Co-author of "Hibernate Quickly"<br /><a href="http://www.manning.com/books/peak" target="_blank" rel="nofollow">http://www.manning.com/books/peak</a>
I haven't really dug into the new parser enought yet to do a valid compare and constrast. I suspect it won't matter much for projects started with Hibernate 3.0, if anything giving more meaningful error messages.
The one time it could possiblybe an issue is when updating a project with lots of existing HQL from 2.0 to 3.0. Nick ran into this a bit while writing the book I believe.