I find that the most useful frameworks provide conventions for structure and code organization along with pre-built libraries for most common functions (DRY). I avoid frameworks that make it overly difficult to deviate from these conventions when needed and those that make it difficult to override built-in functionality. The best frameworks are the one's that realize early on that they cannot solve every every problem and edge case.

For PHP, I prefer to use Kohana. It has a nice structure and does just enough, while maintaining flexibility.