Mavericks and future versions of OS X will be free.
If you weren't sure whether the free upgrade to Mavericks is a one-off thing or not, you've got confirmation now.
Considering Apple isn't likely to introduce major changes every year, this shouldn't hurt their profits that much, considering their margins in hardware. I think they made a good decision because less fragmentation helps developers a lot . The only biggest change in OS X's future would probably be the redesign of OS X with more of an iOS feel to it. Otherwise, they'd be releasing Mavericks-like updates which focus more on the system-level features with some new additions.
Considering Microsoft's software strategy which is to license software, I'd be interested to see how Microsoft reacts to this i.e. whether they will start releasing many small updates per year or still stick to a high-price major release (I'm sure $119/$199 is a bit too much for an
OS update new OS purchase).