Having to buy seat cushions for all your chairs because you're not carrying around as much padding on your own butt anymore!
Discovering new and odd bones, muscles, and dips in your body in general. (See the "how to shave your new armpits" thread. Where did that go?)
Wearing out expensive running shoes from the fancy running store (no more getting the cheapos from the Sporting Goods Clearance section, because you're a
runner now).
Ditto with bras, because the laws of bra engineering apparently make it much more expensive to construct a 34DDD than a 40D. (Yes, you may be the one whose boobs didn't shrink with the rest of you.)
Being at the head of the pack when you're with the hiking group instead of the end, and having people think you know where you're going.
Your hairdresser (who is very opinionated) finally agrees to give you the cut you've been wanting, because your face is a different shape now.
Having the security guard at the field research station, who's seen you every summer for the last 15 years and cheerfully waved you through, not recognize you, and try to make you sign in under the "non-members".
Oh, and I almost forgot: much lower intimidation factor when staring down tank-like Mediterranean grandmas for the last seat/grocery item/flea market deal. I bet you know the ones I mean, Serbrider!