SL is reasonably stable IMHO, but not rock-solid. While difficult to crash, VirtualBox and any other having modules so close to the kernel don't help with stability.
How Mavericks can be more unstable?
Dont ask. constant beachballing on my Macbook Air.
Samba process often getting stuck when doing network file reads/writes forcing a reboot (forced reboot, even shutting down normally gets stuck)
Application Crashes
Higher ram usage.
Higher CPU usage
Audio doesn't always work when resuming from standby.
these are just some of the issues ive been having with Mavericks. For ultimately something that doesn't feel / look any different than it's previous iterations.
I have tried 3 fresh installs.
First using my MBA's system restore to Lion, then upgrading to ML then to Mavericks
Second, Straight from Lion to Mavericks
Third: Straight to mavericks from USB installer.
Same issues everytime.
Lion and Mountain lion do not give me these issues.
for the first time ever in my life, I have to admit that OSx crashes more than Windows... in fact, I have yet to have a windows 8 or 8.1 system crash.