Activities isn't actually anything really new. It's always been available with the Live Toolbar. Though it was called instant information and not as clearly labeled as anything else.
Actually, it's really called Entity Extraction (whatever that means) and it was part of Live Labs a while ago : http://labs.live.com/Entity+Extraction.aspx
About time MS included it by default. As for WebSlices, that's also really neat. I'm still waiting to hear:
1. Skins support
2. Easier to create add-ons
3. Support for other OS's
4. Mobile version (Deepfish/WinMo?)
5. Live "Tab" Thumbnails (think Opera)
6. Most of the goodies long available in IE7 Pro
7. Tabbing History
8. Better inline search
9. View web page source code (think Opera)
10. Fancier looking zoom control (Think Office 07' like)
That would be awesome.