How will WordPress 3.5 affect your average site?

I’m new to WordPress, and I’ve always found upgrading a CMS to be a risky decision. When you have modules that were written a year before you have to wonder if they will still function after the upgrade.

I have a really new WP install. The massive majority of my activated plugins are authored by WPMU. I have a few others like W3 Total Cache & Nginx Helper.

My question is, how will this upgrade affect my site? Will the WPMU plugins still work afterwards? And from your experience, do most plugins generally work with a new version of WordPress barring any severe changes by the WP Devs or short-comings by the developer of the plugin?

And what about themes? It doesn’t seem like this update has made any direct changes that could affect themes, though I’m still researching what’s new.