Amazon’s new Kindle Fire tablet, according to the usability experts at the Nielsen Norman Group. In a report published Monday, Group founder Jakob Nielsen says the Fire suffers from all sorts of usability problems and offers a disappointingly poor user experience.

Nielsen takes issue with virtually every aspect of the device, from hardware to operating system, but his chief complaint seems to be its 7-inch size and Amazon’s failure to optimize the user experience for it.

“The most striking observation from testing the Fire is that everything is much too small on the screen, leading to frequent tap errors and accidental activation,” says Nielsen, who labels the issue the “fat finger problem” and notes that he’s seen it occur on pages with as few as two text fields and button.

Equally troublesome, the fat Web site problem — cramming a site intended for a desktop browser into the Fire’s significantly smaller screen with little or no optimization. “Using designs intended for a full screen on a 7-inch tablet is like squeezing a size-10 person into a size-7 suit,” says Nielsen. “Not going to look good.” Yet, that’s what the Fire attempts to do, even though it does wonderfully with mobile sites optimized for 3.5-inch screens.

Beyond these, Nielsen has plenty of other criticisms. The Fire is too heavy; its screen updates lag; its “Page” and “Text” views are lousy. But again, most of the gripes come back to the 7-inch form factor. In order for the Fire to succeed it needs content and services that are designed specifically for it. Repurposed content from other platforms just isn’t going to cut it.

“A 7-inch tablet is a sufficiently different form factor that it must be treated as a new platform,” Nielsen concludes. “Optimize for 7-inch or die.”

Why would Microsoft pay such a premium for Yahoo, given slowing growth in display advertising? The Nielsen Norman Group’s Jakob Nielsen has an idea or two. Here’s one of them: Web-mail-associated searches.

When they want to search, a lot of users go for the nearest ‘type-in place’ they can find. So if a search box is visible on the current page, that site will often get the query, and the ensuing bountiful revenues. Many users don’t even know how to get to the homepage of their favorite search engine.

“Thus, if Microsoft took over the email accounts currently served by Yahoo, it would get many of the searches done after people check their email (something that’s very sticky–users are reluctant to abandon their email addresses).

“The question, of course, is whether such accidental use of a search engine is as valuable as searches by more-determined users who go straight to their favorite search engine. Yahoo would know, since they serve both kinds of searches, but the numbers are surely kept as a state secret.”