Every layer of abstraction you add, not only adds a layer of complexity and extra points of failure, but also adds restrictions.
By definition, an abstraction is more restrictive ... otherwise, you're not really abstracting anything.
If you're making a simple web app, an ORM is just fine. If you're building enterprise-level software, it is a really really bad idea. Half your code will be using it, the other half will be forced to use half-assed SQL queries that try to fit into your ORM. Because, quite simply, you will need all that SQL has to offer to make things work right. You can't afford to abstract SQL away. Trust me, I tried. In the end, the best you can do is go with something like LINQ.
I built a LINQ like system for PHP a while before Microsoft did it for .NET =)
On a side note, I would also stay away from all the frameworks and build one yourself. If you're on a long-term project, it's worth it. You'll understand what is happening and what each call really costs you. You can also refactor an existing framework. Either way works.
By definition, an abstraction is more restrictive ... otherwise, you're not really abstracting anything.
If you're making a simple web app, an ORM is just fine. If you're building enterprise-level software, it is a really really bad idea. Half your code will be using it, the other half will be forced to use half-assed SQL queries that try to fit into your ORM. Because, quite simply, you will need all that SQL has to offer to make things work right. You can't afford to abstract SQL away. Trust me, I tried. In the end, the best you can do is go with something like LINQ.
I built a LINQ like system for PHP a while before Microsoft did it for .NET =)
On a side note, I would also stay away from all the frameworks and build one yourself. If you're on a long-term project, it's worth it. You'll understand what is happening and what each call really costs you. You can also refactor an existing framework. Either way works.