Not many “must have” new features apart from mash-ups – Photomerge - Compositor. Probably no great rush to upgrade if you already have PSE12 as most major improvements seem to be under the hood, unless you are:
A Windows user - wanting a 64 bit app.
A 64 bit PSE12 Mac user - wanting support for retina display or the slideshow.
Still no 16 bit support so it’s $99.99 (upgrade $79.99) vs $9.99pm for full photoshop+Lightroom.