As always with any major OS release, you have people crawling out of the woodwork that are pinning every problem with their systems on the new OS. It couldn't possibly be the hardware that they're using.

I'm not saying Apple is perfect, but when you've got millions of people downloading an OS - many of which are jumping 2-3 entire major OS versions - even 0.01% of people having the same problem is going to result in hundreds of people with that problem. When you combine that with a public discussion forum run by the company (that's regularly indexed by the major search engines) and a small but vocal portion of the customer base that feels entitled to immaculately perfectly functioning devices because "they bought an Apple computer" (read: luxury) and you end up with runaway threads of coincidence-now-fault.

"Wow, Apple is giving away 10.9 for free? Even for old machines running 10.6? Gee, I've got a 5+ year old laptop with the original battery I haven't used in a while - wonder how it'll work with that."

Saw that someone mentioned the MacBook Air Gen 1 (they probably meant Gen 2 since Gen 1 had 32-bit EFI and maxed out at running OS X 10.7) - which originally came out with 10.5 ... but you could still get 10.9 onto it if you did the USB installer trick. In point of fact - Apple actually added the visual indicators for servicing your battery in 10.6. If people are installing 10.9 over the top of 10.5 (it works if your machine can run 10.9), they will likely be seeing a battery warning indicator for their 5-6 year old machine that has been a valid issue for quite some time (especially since the battery isn't considered user serviceable on a Unibody Mac) - it just never popped up in their face. I would be more believing of this thread if every single person posted their battery readings and all turned out to have amazing low cycles combined with amazingly reduced capacity (which is when the Service Battery dialog appears).

If you find yourself to be in the same boat as the people in this thread, you should probably read this KB article from Apple and immediately have your laptop retrain its battery knowledge:

http://support.apple.com/kb/HT3964

"Intel-based Macs: Resetting the System Management Controller (SMC)"

After performing normal troubleshooting, these symptoms may indicate that an SMC reset may be necessary:

-Power-: [...] The battery does not appear to be charging properly. [...]"