Dmitriy Sintsov (2011-07-29 07:54):
Speaking of server-side (php interface), yes, MediaWiki is largely backwards compatible (except for static call Linker::link() and few ancient things, like wfLoadExtensionMessages‎() and so on). Speaking of client-side (Javascript), it is not so simple. Since 1.17, I am experiencing weirdness with client-side scripts in non-ResourceLoader ready extensions. For example my own Extension:QPoll had minor glitches until I've ported it to ResourceLoader. I guess that is because the scripts are now loaded at bottom, not at the top, or maybe due to jQuery introducing it's own scopes, while old scripts were assuming to have run in window scope. However, QPoll has very little tiny client-side script, while Extension:CategoryBrowser also have glitches and scripts are much larger - I still don't have the time to port it.
I think you shouldn't use mw.whatever stuff just yet (so the code will be backward compatible). Most of the stuff was already available just phrased differently. I think you should use jQuery anyway where you can instead of wikibits functions, though (e.g. $() instead of addOnloadHook()).
As for bottom/top this should probably a problem only for move from 1.16 to 1.17 rather then the other way around. So if you managed to make it work for 1.17 it should work for 1.16.
In contrary to claims that Extension:FCKeditor works fine in 1.17, I've experienced opposite with my own Vector-based skin, not Monobook skin. Of course, I might try again, however I really disliked the way the extension is coded - with assumption that Monobook skin is being used, unreliable tricks with Parser and so on.
From what I've seen the problem is not a Vector skin itself but the new wikieditor (editor toolbar). If you will disable it it should be fine.
Regards, Nux.