Collaborated from this NG (mostly Paul Smith):
1) New fonts, much easier on the eyes.
2) Restore previous versions of files (volume shadow copies).
3) Far better security, user rights being used, address space randomization
on boot etc.
4) UI rendered through DirectX (no tearing windows anymore when you drag em
around!)
5) Search, being able to save search results as virtual folders, network
searching.
6) Offline files that take seconds to sync, and does it without annoying
you.
7) Windows Mail has these groups all set to go.

8) Diagnostics, loads of them can keep you busy for ages.
9) IPv6 and IPv6 over IPv4, brand new network stack, makes communicating
behind a NAT easier.
10) New faster audio stack.
11) New driver model, puts more drivers into user mode so they don't bring
down the system if they crash.
12) Ad Hoc networking support, in Meeting Space and available to 3rd
parties.
13) Photo Gallery, plus image tagging that isn't in a separate database but
stored in the files themselves so you don't need to retag them on different
machines.
14) Networking is easy to set up now.
15) Parental Controls, great for locking down what your parents (or
children) can break on their systems and for seeing what they've been up to.
16) Multiple clocks.
17) Sidebar.
18) Superfetch pages applications into memory so they start up instantly,
ReadyBoost and ReadyDrive too.
19) Speech recognition now built in.
20) Tablet features now built in, you can even use a regular drawing tablet
to unlock the hand writing recognition and all that.
21) Instant search feature. Find files in a flash.
22) WinPE 2.0 (based on Vista) allows hot plug n' play of USB devices (among
other improvements over previous versions of PE)
23) Windows Easy Transfer
24) New media tools like DVD maker, etc.
25) Parental controls.
Before upgrading, you need to make sure you have proper drivers for ALL your
hardware. Unless you plan on buying a new machine. Either way, if you need
help with any of that, just holler!