To be sure, I think OS X 10.8 Mountain Lion is the best OS Apple has ever released. It feels like a completed version of what Mac OS X 10.7 Lion should have been. It's fast, it's clean, it melds some iOS concepts to OS X and, for the most part, it just works.
That being said, no OS is entirely perfect and there are some gripes my colleagues an I have about Mountain Lion. We've compiled them into the list below. Now, most of these gripes are minor and they'll probably be corrected in future updates like 10.8.1 -- at least, we hope so.
After reading through the list (which may grow) feel free to add your own gripes in the comments. And please, don't let this list of minor gripes put you off the OS. It really is the best one Apple has ever released.
- Wobbly scroll on external monitor.
- No "Reply to sender."
- Signature sticks when changing accounts (Exchange only).
- Accepts invitations in the top calendar, not the default Exchange calendar (and not the account that received them).
- Mirroring only.
- @mention tweets don't show from people you don't follow.
- Clicking on an email message in Notification Center should take you to that specific email's inbox in Mail, NOT the universal inbox for all your different email accounts.
- Under Safari>Preferences>Passwords the list of websites and your usernames should probably be hidden until you enter your administrator password. The passwords are already hidden by default but it seems like a security problem when anybody using your computer could see what websites you go to and what your usernames for the websites are. Knowing your usernames gives them one less thing to hack. Not to mention it tells people exactly which sites they should target you at since they know you're a member of the sites by looking in Safari's preferences.
- Loss of RSS button in Safari means users have to jump through several hoops to subscribe to feeds. They also have to spend money to buy Reeder for Mac (or another RSS reader).
- You can no longer rearrange the order of stocks.
- Stocks don't sync with Stocks app on iOS devices (they never did, but they should).
- Numerous graphical "tearing" issues throughout the OS on the MacBook Pro with Retina Display. Most prevalent in TextEdit and Mail.
- "Open Recent" has disappeared from opening dialog splash box in many apps, most notably iWork apps.
- No "share" button in TextEdit.
- "Save As..." behaviour is completely contrary to how it worked pre-Lion.
- MacBook Pro with Retina Display has serious wake-from-sleep issues. Requires hard reboot more than 50% of the time.
3rd party apps:
- Microsoft Office apps refuse to hide.
- Sandboxing (or something else) seems to be preventing the iStat Menus Dashboard widget from delivering data on CPU processes.