Designing a Plugin Framework for a CMS

[eluser]CodeIgniterNewbie[/eluser]
These questions a very general question about how one might architect a "plug-in" framework for a CMS. This doesn't even have to apply to what you would do if you were developing the plug-in framework in CI.

So:

1. How would you design a plug-in framework?
2. What features would you want your plug-in framework to support?
3. What features implemented in other plug-in frameworks do you not like?
4. Which existing plug-in framework have you found to be the best designed?

If anyone has any good references designing plug-in frameworks, please post your reference. Thanks!

[eluser]GlennJ[/eluser]
I know this is almost a year old now, but I too am looking to make a plug-in framework, but for an e-commerce system instead.

Could anyone give some ideas on how to implement this in CI?

[eluser]GlennJ[/eluser]
Found this thread which answered all my questions

http://ellislab.com/forums/viewthread/87350/P0/