Mountain Lion is fine. Keep running Snow Leopard to the end of time.
While I have not experienced much in the way of bugs, as an OS X developer since 10.1 I can assure you 10.7 and 10.8 have a full list of open and known bugs still yet to be addressed.
Examples:
- WiFi connectivity - many users reporting dropped and/or inconsistent WiFi (clean installs do nothing while 10.6 on the same system in the same conditions works flawlessly)
- OpenGL Core - poor at best, with little support for 4.0 if at all, well behind others
- Memory Management - while improved in 10.8, still a far cry from 10.6 and earlier, even with 4-8 GBs RAM
- Networking protocols - since 10.7, AFP and SMB have been a nightmare
- Lack of full server features - even dropped necessary features available in 10.7 and earlier
- Myriad of graphic issues, especially involving rMBP's
- Finder needs a serious overhaul - HFS+ has been showing its age for a while
- iPhoto/Aperture "Time Machine" support - removal of full "Time Machine" support since version 9 - previously one could open iPhoto and restore deleted pictures as can be done in Mail/Contacts/Calendar, now one must restore the entire iPhoto or Aperture database is the recycling bin is inadvertently emptied which is more common than you would believe
- "Mission Control" - discussed ad nauseum - poor implementation of a previously successful workflow w/ Exposé and 3D Spaces, many resort to third party app's such as "TotalSpaces" to restore a once beautiful workflow, "Mission Control" is wasted graphical eye candy that taxes system resources
On and on. Apple has their collective arses up their iOS butts, more focused on Facebook and Twitter social networking integration and less focus on a solid OS X experience. God I miss
Bertrand Serlet, since he left after 10.6 OS X has seen a drop in overall quality under its new "leadership". I haven't seen so many opened and unresolved bugs with OS X developers in all my years, much more than 10.6. Many wish Apple would go back to 10.6 and earlier beta releases, releasing a beta.dmg every 2 weeks and requiring a fresh install each time. This certainly seemed to alleviate any issues with DP's releases over existing installs.
Moving OS X to an annual release date to match iOS is a big mistake.
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I can honestly say I want almost none of these.
Then you're not a developer as some of those are SDK's, and I would love to have those available.