Sgt_Kickaxe - 3:55 am on Dec 29, 2012 (gmt 0)
My 2 cents, barring a manual penalty it really feels like you have the ability to drop in rank quickly with even some mundane changes but not the ability to regain lost rank until a major Panda/Penguin type update is complete.
A template change, the addition of a forum and comment system choice all seem to be able to tank rankings quickly if the site has been otherwise stable. I'm not sure that's working as intended since the quality of your content did not get worse, it in fact improved.
What changes is Google's level of trust in your site.