First, BACKUP your computer before you upgrade. It is always smart. Yes I work for a backup software company, but I want to keep the blog objective, so I won't mention the company name.
Overall I think Mountain Lion is a nice upgrade from Lion, but a couple of changes in the system may be confusing to users or make you unhappy overall.
1) Safari no longer supports RSS feeds. If you are like me, I check RSS feeds for about 100 websites every single day, multiple times per day. Seeing the RSS feeds stop working suddenly was a shock. I had a little bit of a panic attach because it would totally change my web browsing experience.
I have come up with my own workaround, that seems to be doing a decent job.
- Setup a Google Reader account
- Copy all of your RSS feeds into the Google Reader account
- Download a program like "Caffeinated" from the OS X App store. You can search on "google reader" to find free and paid RSS feed programs. Caffeinated seems to be working really well for me
- Login to your Google Reader account within your RSS Reader and all of your feeds will show up.
2) Gatekeeper: Apple has added a new security feature that blocks programs that do not have an official Digital Signature for Mountain Lion.
If you have a program downloaded from someplace other then the App Store, that program may not launch until you make a System Preference change. In my case, non of my Safari Extensions would work.
The above change will allow all programs to run, like they did before the upgrade. All developers will eventually release updates with a correct signature to avoid this issue.
3) Messages. If you want to see your buddy lists, so you know who is online, you must use "command-1" on the keyboard to display the list of online users.
4) The Google search field in Safari is now gone. It has been combined with the address bar. Many of us are not happy about this, but you will get used to the change.
5) Safari started opening all Windows in a new tab and not a new Window. I prefer new windows. You can go to the Safari Preferences>Tabs and change the drop down menu to "never" as a fix.
For me, these were the biggest issues I experienced on 3 different computers. I hope it helps you.