I hate I had to download it from the Apple App Store because it took over an hour to download it where I bought the USB stick version for Lion which loaded quickly.
It sandboxes all applications where some take a little longer to load because it conflicts with the new rules about applications working within their own space and not using other applications.
This is important if an application accesses, uses or load other application. OS-X won't let you anymore, or at least only in a very restricted manner, such with QuickTime.
Apple's software update method through the App Store now sucks. I liked the old update tool through it's own window separate from the App store. I hate this new way because I've moved three apps off the App Store but you can't remove them from the App store and they all say I need to update them, even though the App Store version is long out of date and obsolete.
So here's my answer to Apple on this.
Let users remove applications from the App store!
As for other stuff, I removed or turned off all the notificatioins because I have third-party applications which do the same thing and better. I don't need Apple telling me too. I haven't checked out the various uses of Facebook and Twitter in applications, like Safari, because I use them from other applications.
I haven't tested all the Adobe applictions because I have a lot of them from Creative Suite (CS) Web Premium 3 through 6 with other CS applications. I lost CS 3 with Lion but had all the rest and the same seems to be the case with Mountain Lion, but some Websites indicate some CS5 and CS6 has minor to some problems. I'm happy Go Live 9 works (my favorite Web design application).
Safari is faster loading, but Apple did one thing many people, like me, hate. Apple removed Java from Safari and all browsers. Now you have to download it from Oracle, found here. I miss the old activity window which is still gone under Safari 6.
Mail has some new stuff. The new Calendar size is too big for me and you can't adjust it smaller, which sucks. I don't need big. I don't like the light for the dock bar when an application is open. I like the old light.
I haven't turned on the backups yet. I need to reformat the Time Machine now because it won't access any Lion backups and is wasted space. I have one OS-X 10.7.5 clone and a complete backup of all four HD's, which I'll turn on later.
I'll add to this post as I find stuff that breaks, goes beserk or I don't like.