While I've always been a casual fan of Brit at the most, I think the part of the reason people are chanting 'Britney's back' like choreographed zombies is because, for whatever reason, she's been heralded as heir to Madonna's throne (blasphemy!).
Truthfully I would nominate Pink or Christina Aguilera, who have continually reinvented themselves musically and visually over the years, and actually seem to have a say in their careers. Meanwhile, Britney's stayed on the 'I'm-so-hot-shaking-my thang-in-the-club-phase for pretty much her entire career. And while in the past you could at least count on ol' Brit to serve it up live, she just doesn't seem to have the fire in her eyes anymore. But hopefully she'll work her way back (I guess that's the optimist in me:).