DC's Power Girl has had plenty of costumes over her history as a crimefighter, but one of her best-suited for fighting evil supervillains was also one of her most hated by her many fans. Costume changes happen frequently in comics, but fans get rather attached to existing designs - or rather, designs made rather infamous by Power Girl's first major artist Wally Wood. Out of Kara Zor-L's many costumes, her look from DC's infamous New 52 is by far the most controversial.
In 2011, DC Comics ended their Flashpoint crossover story with a universe-changing relaunch of the entire line of books. Dubbed the New 52, this new initiative revamped every major superhero. Long histories of characters were retconned out of existence, while others simply received new costumes and looks (Superman lost the red trunks and gained a collar on his suit, for example). Power Girl was no exception...but while some fans welcomed the changes made to other heroes, the same could not be said for Kara Zor-L.
Power Girl's new costume was certainly a major departure from the white leotard with blue accents (belt, gloves, boots, etc). While the majority of the suit is still white, a stylized yellow "P" logo commands attention on the left side of the costume; it notably does not share the same design as Superman's famous S-shield. The logo was surrounded by a large red border, which spread to Power Girl's shoulders and extended downward as a cape. The blue gloves were integrated into the body of the suit. This was also one of Power Girl's first major outfits in years that closed off the infamous "chest window", covering her completely.
This outfit was far and away one of the best superhero costumes Power Girl has ever worn, from a practical standpoint. Crimefighting in leotards and exposed skin isn't necessarily tactically advantageous, even for a Kryptonian, and . However, fans reacted negatively towards Kara's new costume and strongly desired the return of the old suit. Perhaps they thought the P was a bit too stylized, and the off-centered logo didn't help matters at all.
It's worth noting that an actual reason for Power Girl wearing her classic uniform has yet to be canonized. Writer Geoff Johns wrote a monologue for Kara during his run in which Power Girl admitted she desired a symbol - like Superman's - but she couldn't think of anything that would "close the hole". Regardless of any explanation, Power Girl's classic costume is back after the New 52's end, and it won't change anytime soon.