Overall system response - including the resistive screen which e.g. frequently misinterprets list swipes as clicks, aspect switch issues, etc
Media Player - in fact, everything to do with media, period.
Image Management (ditto)
Camera (And yes, I count BlessN900 - the results of which are for the most part still worse than an iPhone 4)
Flash videos (go to e.g. Dailymotion and compare the jerkfest on MicroB vs Skyfire on iPhone)
Navigation (in comparison to Android and even Symbian)
SSH / Remote Access (from your phone. Only the painfully nerd need SSH access TO the phone - what you actually need is remote wipe. And the N900 has that, right?)
Network Management
Games
Personal Productivity (DocsToGo and... Bueller? Bueller? Openoffice on Debian on a mobile form factor is a 24-carat joke, and about as /un/usable for everyday duties as accessing Office running on a PC from the N900)

...I could go on for, well, quite a long time.

Now, would you take booting a half-assed implementation of other mobile OS's, CLI and customisable UI, a giant body with no accessories - not even a wired full remote that works - and chronically short runtime once you start running some poorly written / perpetually-in-alpha apps (which are really common on the N900) over all that? Then you are a candidate for the N900.

It's a shame - but the Nxxx track record should have been, and indeed has been, an excellent indicator of where this is(nt) going. More resources spent on making Symbian relevant nowadays rather than being sidetracked by a forever-experiment-in-progress might have boded better for Nokia.