Jani Hartikainen looks at a certain something that's missing from several PHP frameworks offered today - a good library for working with the M (Model) in MVC:

While ZF provides data access classes in the form of Zend_Db and related components, it doesn't provide any concrete examples of how you would implement a model class. This often confuses especially beginners. Providing a common base model, or an interface for implementing models, does have certain benefits, but it also has some problems...

He defines what a good model interface should be and do before looking at the good and the bad of implementing them. A good reason to have an interface is to have a common method the classes could use rather than just using a database wrapper class. On the downside, this can also be very limiting and require you to be locked in to how that interface works without the flexibility you might be used to with a looser framework.