Take the iPhone's Safari browser, for example. With no dedicated navigation control (the BlackBerry has a pearl-sized, clickable trackball), Web forms are tough to fill in.
By contrast, the BlackBerry's trackball works like a tab key, sequentially highlighting clickable controls and scrolling to bring them into view. With the iPhone, buttons and hypertext links must be tapped on, which often requires zooming in far enough to make the target the size of your fingertip.
The iPhone has no options to set the default page size or to make text larger, so zooming is a frequent necessity. The iPhone makes zooming easy for read-only browsing, but forms are a bear. For interactive content, Safari works best with sites specifically designed for it.
Working across apps and documents
The BlackBerry has a single buffer for copy/cut/paste operations that works across applications. You can't cut, copy, or cut on the iPhone.
Unlike the iPhone, the BlackBerry allows several applications to run at once, so you can pull text from a note to an e-mail message, paste text from an e-mail message to a Web-based blog entry interface, and so on. The BlackBerry's paste buffer only supports unformatted text, and for the most part, that can be said of the device as a whole.
The BlackBerry -- at least the 8800 and 8820 devices I used -- has inadequate, borderline useless support for rich document attachments to e-mail through painfully slow server-side rendering. HTML e-mail is not rendered with formatting intact.
As you'd expect, the iPhone is all about visuals. With Safari embedded into the mail client, HTML e-mail snaps to the screen as fast as text. Viewers for PDF and Office/iWork documents are intrinsic to the mail client. iPhone users are set up to expect a desktop-like experience with rich e-mail attachments, but the iPhone doesn't yet deliver.
I worked most often with the iPhone's PowerPoint/Keynote and PDF viewers. They rendered the first page of most documents fairly quickly, but navigation within a document was slow, and the viewers consistently crashed or went unresponsive for unpredictable periods of time while navigating within documents over 20 pages or so. I do prefer an unstable rich document viewer to none at all, and when the viewer is functioning, its rendering is bang-on.
Other usability issues
I have just enough time to rattle off some other usability contrasts, starting with media. iPhone is, after all, an iPod. It's got widescreen, which traditional BlackBerry devices don't deliver. The display is beautiful, and everybody with an iPod can push their entire iTunes library to their iPhone. That's at least half of the appeal.