This is how you change between applications and start new ones. So what is so bad about it? Well, it really kills your flow when you're working. Rather than clicking on a shortcut to start a new application (you can't put icons on the desktop in Gnome 3), you have to bring up this activity center and find what you want. It really interrupts your work flow to do this. It takes over the whole screen and pulls you out of what you're working in.
Issue #3 Customizability: Gnome 3 is a lot more difficult, by design, to customize to your liking. I mentioned no desktop icons. But it's also fairly difficult to change the color themes. You can't customize the top menu at all. If you want to hide the battery icon or accessibility icon because you don't need them, that's just too bad. I searched in vain for a way to hide those icons and came across this gem of a page which describes how little you can customize Gnome 3. Fonts also look horrible and aren't easy to change.
|Gnome 3 menu|
For another example, the original Gnome 3 user menu didn't include a 'Shut down' option. You had to log out to the main screen to shutdown your computer. Really? I have to log out, wait for the login screen to come up, and then shut down? Based on user complaints, the developers added an option where you can hold alt to change the 'Suspend' option in your menu to 'Shut down.' Who's going to know that exists?
|Gnome 3.2 menu|
I gave Gnome 3 a good try. I wanted to like it, but it's just all wrong. I think this is one reason why Linux doesn't have a better following than it does. You get a lot of projects like this where the default is just ugly and unusable. It's not worth the time to put into customizing it to where you can stand to work with it. That's where Mac OSX and Windows excel. They're decent to look at and are actually usable by default. Being commercial applications, I'm guessing they took user feedback into account and have avoided the Fundamental Gnome Error.