Usually, there is an entry on the devblog of a new feature before releasing the patch which includes it. In the case of the new leveling and spell system, the entry didn't get posted because of the will of rushing the patch release this season. It will appear any day now, showing in detail what there is and what there will be, but until then I'm afraid you will have to be in the dark (unless you play test it and see what is all about). There is some info in the description of the patch on moddb (and e-mail announcement if you were subscribed on the day of the release), but that's a really short summary.
My apologies,
Patrick
EDIT: I have posted a new entry regarding this on TPC devblog: http://the-protectors.webs.com/apps/blog/show/23121661-new-feature-hero-leveling-overhauled