EDIT: An even better solution is just to hold option as you double click the video in the activity window. The method below still works but this gives you the exact same functionality as in Snow Leopard.
I experienced this as well, but have found a solution. It's not as elegant as in Snow Leopard but it works:
If you just want to watch the video without flash's laggy performance and loud fans, especially HD stuff, steps 1-5 are all you need. To save the video to your computer, follow 6 and 7 as well. Enjoy.
1 Double click the video in the activity window to open it in a new tab
2 Copy the URL of the new page with just the video in it
3 Open QuickTime
4 Click File > Open Location (or press cmd-L)
5 Paste the URL of the video - it will now open in QuickTime
6 Click File > Export (or press cmd-shift-S)
7 Choose a location and save.