Relax everyone, it's only the first beta!
All kidding aside, it's nice to get such excellent feedback so soon after the release. Let me see if I can cover all of the issues raised here.
Auto-hide animation: This is planned for 3.2.1 final.
Taskbar shown after Windows taskbar: If you are referring to the new auto-hide mode, this is by design. There will be a bit of a lag until the Windows taskbar is shown, but it works as expected.
Auto-hide taskbar while menu open: It would be possible keep the DF taskbar visible while the DF menu is open, but DF can't detect when the window context menus have been closed, so it's not really possible to keep the taskbar visible while they are shown.
Show when empty issue: When the Windows taskbar is not set to auto-hide, I would recommend setting your DF taskbar to "Auto-hide enabled".
Grouping icon clipping: Thanks for letting me know, I'll have this fixed up for the next version.
Show context menu focuses the window: This has been in DF for a long time, and it's something that DF has to do. In order to show that context menu, it has to be the currently focused window, sorry.
As for the preview windows, I knew this would be reported.
I want to get the Windows XP/Vista grouping styles completed first, as they're the easiest and still work perfectly in Windows 7. After this is done, I will work on the more advanced side-by-side window previews, and give you the option to use the simple list style, or the new preview style, even in XP/Vista. Don't worry, it's all coming soon.