When I started writing this post, two weeks ago, 46% of the top 95% most used Firefox extensions on AMO (according to Firefox Add-on Compatibility Report) still did not manifest 3.5 support. The Mozilla team has been checking in with at least the topmost few of those projects (being one of the Greasemonkey maintainers, I got one of those pings, a few weeks ago), about whether we were having any issues upping the max-version of their extension to 3.5. We had not tried, at the time, but (happily) did not see any big issues, so we just let the release already out claim 3.5.* working.
For other extensions, like MashLogic which I hack on at work, that transition was not as trivial. Our integration surface towards Mozilla being larger than Greasemonkey's, more things changing underfoot get a shot at breaking things, and did.
The information about backwards incompatible changes available at MDC was and is still a bit sketchy, and did not cover issues we were hitting, at the time -- it seemed XUL documents were getting XPCNativeWrapper treatment in 3.5, that they did not have in 3.0, so we had to start adopting Greasemonkey-like roundabout practices to read and write properties of the XUL DOM. That's the kind of information that is good to be able to read about -- and there, specifically.
It's a wiki, though, so if you find something that isn't already there, while you are scratching your head about why things are not fine and dandy in 3.5 (due rather soon), please add your findings too. I did a brief writeup on my own findings.
For MashLogic, we actually ended up removing the xul page we had used for configuration to do the same thing in HTML instead; work we had already mostly done for our IE port, anyway. Other tidbits remain here and there, though, before we can claim having fully working functionality under 3.5.
On top of that, there is the not so enjoyable multi-week delay at AMO, getting our latest release through their review system. Our current release is thus so far only available if you look at their see all versions page, and grab the so-called "experimental" build on top. User reviews seem to help get the ball rolling a little, though, so if you try it out, we're really glad if you share your opinion with other users there by writing a review.
Whether you are a user or extension developer, sharing your experience like this really helps others. Especially if you do it where others go looking for it. MDC and AMO are rather good funnels for developer and user centric Firefox extension related things, and the more we use both, the better they get. Thanks for your help!