Both the iPhone and Windows Phone 7 have capable Contacts apps, but the iPhone 4 makes it easier to navigate through your entries. You can jump to names by tapping a letter at the side of the screen, such as "t" to get to people whose last names begin with "t," or seek quickly for someone in the Search field by typing in part of the name. In Windows Phone 7, you have only the search capability to find contacts; there's no quick-jump function.
Windows Phone 7 lets you designate users as favorites, to put them in the Home screen. The iPhone 4 has no equivalent. And Windows Phone 7 lets you link contacts, so you can see all their information in one place, such as personal and business entries for the same person, or separate entries for family members. The iPhone supports email groups, but you can't create them on the iPhone; they must be synced from your computer's contacts application. Windows Phone 7 has no group list capability.
The winner: The iPhone, thanks to its support of critical Exchange ActiveSync policies. If you don't use Exchange, the two mobile OSes are fairly equivalent. The iPhone is slightly better in its email handling, but Windows Phone 7 is better for calendars. Contacts management is a draw.
The iPhone provides more useful apps than Windows Phone 7 does. Both provide email, contacts, calendar, browser, calculator (except on the iPad), maps, a music player, photo display, a video player, multi-user gaming, and SMS messaging apps. The apps are equivalent in most cases. One exception is the photos app, where the iPhone supports albums and Windows Phone 7 does not. Another exception is the maps app, where the iPhone provides satellite views in addition to cartographic ones; the iPhone's maps app is also much faster at returning directions.
The iPhone provides several apps that Windows Phone 7 does not, including those for a clock, the weather (except on the iPad), stocks, voice memo, and YouTube. Although Windows Phone 7 supports alarms, it offers only a subset of what the iPhone's clock app does.
Windows Phone 7 has a set of apps called Office: Word, Excel, and OneNote. But don't let the Office name fool you -- Word and OneNote are very rudimentary apps, good for basic notes entry and extremely light editing. For example, tap and hold a word to select it; from there, you can make it bold, apply a colored highlight to it, or add a note -- but you can't select a range of text. You can't choose fonts either, though you can apply numbered and bulleted lists.
Excel likewise is good only for very basic editing; constructing formulas is very difficult, as you can't tap a cell to enter it into a formula. You can tap the fx icon to get a list of formulas, as in the desktop version, but the default keyboard for Excel doesn't display two of the most common symbols used in formulas: = and *; you have to switch to a second symbols keyboard.