WoWInterface - What do you want in an addon API?

I've mostly been playing RIFT lately. RIFT does not have an addon API.

...yet.

In a couple of interviews Trion devs have pretty strongly hinted that there may well be one. Their official stance is "we will, but only if we can do a really good job of it".

That said... The Trion devs have been really, really, good about listening to feedback. We actually got our appearance tab feature. Idle suggestions like "please change the wording of this button" actually get done in future patches.

I am looking for feedback from addon authors as to things they would like to see in an addon API. What about the WoW interface do you like? What do you dislike? What do you wish it had? What parts of it always rub you the wrong way?

Just to be totally clear:

1. I am not a Trion developer, employee, or anything like that. Just an enthusiast.
2. There is no formal statement from Trion about addons, so it is conceivable that this is a pure dead-end.

On the other hand, I've never met a programmer who didn't enjoy talking about what a language or interface should have been like, with or without any actual expectation that this would influence future events.

Please understand, my intent is to take posts from this thread and forward them to Trion, and/or just point them at the thread.

(Yes, I know this isn't "about WoW". Thus the off-topic forum.)

I briefly considered some boilerplate about how I was posting this at other WoW addon development sites, but then... I thought about which addon developer community I actually got useful help and advice from, and decided to just ask here. This may not be all of the WoW devs in the world, but the concentration of good devs here is pretty noticeable.

(Please, no pro/anti RIFT stuff. The question is "what would you like to see in a hypothetical addon API", not "which game do you like better".)