Office XP adds a variety of features on top of Office 2000, but maintains the same file format as both Office 2000 and 97. Most documents will be both forward- and backward compatible between versions, which I personally like.
New to Office XP are Task Panes, basically overgrown toolbars that do not hide your document or steal the focus from it. They include styles, foreign language translation, and a startup screen that provides a few quick shortcuts if you don't want to do anything with the default blank document that appears. For people who type in Japanese, a new Japanese greetings toolbar is available with access to dozens of temporally appropriate salutations and standard phrases. Benri da ne.
Many Slashbots wince in horror at the mention of Smart Tags, but they are in Office XP. When you type something of interest such as a name from your Address Book or a stock ticker symbol, a small icon flashes nearby. If you click it, you can choose to import appropriate information such as the contact's address or the stock's current price. In Microsoft Excel, you can set up a rudimentary portfolio view this way, using MSN MoneyCentral to keep prices up to date. The tags are not very intrusive, although they are animated to call for user attention. They, of course, can be disabled.
However, I take eXcePtion with the notion that the Office Assistant is gone from Office XP. While it doesn't show up immediately or offer advice when you start typing a letter, it most definitely does appear when you hit F1. A combo box for typing questions appears in the menu bar, eliminating the primary function that Clippy performs.
The only way to prevent the appearance of the Office Assistant remains unchanged from Office 97 and 2000: DON'T INSTALL IT.