There's a lot of new feature in 4.2 but here are a few of the highlights:
Promoting Content - A new way to promote content in your Community internally, on Facebook, and on Twitter.
Clubs - Clubs are a brand new way of supporting sub-communities within your site. Many people have requested social group functionality in the past and Clubs are our implementation of this concept.
Reactions - Offer more fine-grained sentiments towards content than a simple up/down or 'like'. They are now in common usage on social networks, and so users expect to be able to be more nuanced in their response to something they see.
Complete Your Profile - Encourage or require members to fill out the details on their profile. Also now allows for quick registration to encourage joining.
And a whole lot more..