(No, PHP, your ass doesn't look fat in that. Honest.)

• Classes have interfaces, which declare which functions/methods it supports.
• A good object is atomic. It does one thing, and one thing well.
• Focus on making a good data object (pizza, cat, node). They don't have functionality. They just exist.
• Behavior objects are active, configurable, and do things.
• Be smart about how many dependencies you hard-code in.
• Every function call or static method is a hard dependency!
  
• Good objects are opaque: it doesn't care what happens on the other side of the function. It should just know that it will get back a string, integer, whatever
• Drupal has a lot of circular dependencies, and this is a problem. Separation needs to happen. Doing so makes the code more testable and swappable.

So what should we actually do?

• Modules should become namespaces. Get out of the hook business. (We think.) Turning hooks into objects is not the way.
• What about the major systems, like theme, menu?
• Magic callbacks: lazy OOP without class

Good practices:

• Minimize extending
• Always provide an interface
• No exposed properties (if you expose a property, you don't know who has mucked with it)
• Don't be private (learn the difference between 'protected' and 'private' - properties should always be protected. methods should be public or protected depending on their needs)
• Constructor injection (don't just call out for a new object - pass info to it.)

Good for learning:

• martinfowler.com has a good article on constructor injection.
• Zandstra, Mark: PHP Objects, Patterns, and Practice (2008) Ignore everything it says about PHP 6
• planetphp.com
• garfieldtech.com - there's an OOP section