Design goals are focussed on run-time performance first and foremost. Applications developed with Inphusion only ever load the bare minimum of the framework at any one time. Classes are loaded dynamically as required and even then rarely-used functionality is loaded separately via a decorator pattern construction.
Simplicity is also a key consideration for Inphusion. There are better frameworks out there - take a look at WACT for example - but they aren't easy to pick up for the many PHP developers who perhaps don't have a high level of formal training or experience in software development. Inphusion mirrors my own experience - I had the initial idea three years ago, but back then, I didn't have the skill to implement it. In fact, Inphusion has been through six complete re-writes as I've learned, sometimes painfully, about a "better way" to accomplish something. So in all this learning, Inphusion isn't about adding an extra layer of complexity upon a "developing developer." It's supposed to make life easier, and if it doesn't then I've missed the point! Anything you learn while getting to grips with Inphusion should be 99% standard stuff that you just haven't encountered before.
Finally, generality is important. Inphusion doesn't do anything on its own. I've consistently resisted the temptation to "applicationise" the framework - give it a Forum, a Content Management System, a Groupware suite or whatever. Inphusion is (I hope) a great way to do all those things, and a whole lot more - but that's not its focus.