The first of these two is a serious issue IMHO (so, much more than a mere annoyance). I hope Microsoft looks hard at fixing this one for us Tablet users!

When UAC is enabled on a Vista machine, you switched to the Secure Desktop to authorise privileged actions (like changing system settings, or installing software). The Secure Desktop is a separate window station. For most users, you'll already be an Administrator, and your token will be sufficient to authorise the action. Just click the OK button. If you are running as a regular user, you'll need to enter credentials of a user that has sufficient privileges.

So what's the problem? The tablet input panel (TIP) isn't available on the secure desktop! If you are using a Tablet in tablet mode, you need to switch back to normal laptop mode to type in your credentials. If you are using a slate, you'd have to attach an external keyboard. That is an issue IMHO. It's covered in Microsoft KB article 927843 (The Tablet PC Input Panel is unavailable when it is running in Admin Approval Mode in Windows Vista) and also in KB article 927838

The second is the removal of IP over IEEE1394 (Firewire). In Windows XP and Windows Server 2003, if you had a firewire port you'd have an additional network adapter. You could network two laptops (or other machines) together using Firewire. As an added bonus this would run at about 400mbps. This was handy if you were running a lot of Virtual Machines (like me) since it gave you another discreet network between VMs running on two different laptops.