The problem is that a HDD change in the One voids the warranty. I mean, if you want to do it after you've had the console for a year, I suppose there's nothing to be concerned with.
However, they allegedly went with SATA II connectors for the new consoles, which can throttle the speed of a SSD. It doesn't excuse going with a 5400-RPM drive, rather than a 7200-RPM one, though. The funny thing is, the 5400-RPM drives on the market (specifically, the 1-TB Western Digital Green) are more expensive than the 7200-RPM ones (specifically, the 1-TB Western Digital Blue). I'm not sure if cost was the reason, but that seems likely.
What's really dumb is that they went with 500 GB of storage. I mean, no one buys less than a terabyte anymore, because it's so cheap.