Those features include: audio commentary (by a number of participants, including director David Frankel and screenwriter Aline Brosh McKenna), deleted scenes with optional director's commentary, a gag reel, and a pop-up trivia track (on Blu-ray only). The DVD carries over five featurettes from the 2006 edition: "The Trip to the Big Screen," "NYC and Fashion," "Fashion Visionary," "Getting Valentino," and "Boss from Hell."
The Devil Wears Prada also stars Anne Hathaway, already an established star, and—in her breakout role—Emily Blunt. It's a saucy, sassy film, with Andrea Sachs (Hathaway) making her mark after a rocky start at Miranda's Runway magazine. Definitely worth checking out, but if you already own the previous Blu-ray edition, this new one isn't necessary (or even recommended) by any means.
Luckily the SRP is low (look for deals, I'm currently seeing the BD at $7.99 and the DVD at just $3.99), so if this isn't already part of your collection then go for it.