When it developed Windows Vista, Microsoft seems to have focused on under-the-hood changes at the cost of some superficial polish. Take a look at the antiquated font installation dialog and the odd mixture of new-and-old control panels (like the Windows XP-style "Display Settings" window), and you'll see what I mean.
Windows 7 is still a long way out, but screenshots of the Milestone 3 build suggest Microsoft has been spending more resources on interface spit and polish this time. ThinkNext.net has a whole screenshot gallery up along with a couple of videos showcasing Microsoft's latest work.
The screenshots show an updated Start menu, a new calculator, a tweaked Windows Explorer, new control panels, and updated versions of Paint and WordPad. Surprisingly, the latter two now tote Office 2007-style "ribbons" instead of traditional menu bars and toolbars. ThinkNext.net says Windows 7 serves up far fewer User Account Control prompts than Vista, too, and Vista's side-bar "gadgets" have migrated to the Windows 7 desktop.
Judging by ThinkNext.net's videos, Microsoft has even made big changes to the calculator. Not only does it have a more modern-looking user interface, but it comes with built-in tools for unit conversions and time calculations. (In the clip, the calculator works out that the difference between March 27, 2001 and June 16, 2008 is "7 years, 2 months, 2 weeks, and 6 days.") Also, Windows 7 apparently includes a lightweight version of Windows Media Player that starts up almost instantly.