WebApps — treats online services as if they are desktop apps (Gmail, Twitter, Facebook)
Do. Not. Want.
Well maybe you don't. But millions of people use gmail, and some of them use ubungu. I know several people who have made the jump of forwarding all of their email accounts to gmail and using that exclusively as a mail client, because honestly it is a better client than the desktop alternatives (thunderbird/evolution/kmail). Making gmail a full-fledged citizen on ubuntu means it can behave just like a desktop app, with a gmail icon in the launcher, notifications arriving together with those from other applications in the system, etc. I for one am looking forward to this feature.
Dash Preview — right click any icon, get a detailed preview of what it is
Why? Should this not be the job of the file manager? Doesn't it already do this?
Well, maybe you're not searching through your file system. Maybe you search for an application to install, and can see a screenshot before clicking. Maybe you are searching for a song in your music collection, you get a preview of the album art, and a button to enqueue it or start playing it. And so on, with many third-party extensions likely to be coming. Is this useless eye-candy? maybe, but it is a lot more than the file previews in your file system browser, and I bet that after a bit of experimenting and tweaking some cool stuff will come out of this.