a) It's not a phone
b) Music Player takes up to 25% of the CPU. Application Manager can use quite a lot of resources, too.
c) My N900 sometimes gets very slow and pretty unusable, so I have to reboot (probably memory/swapping or Tracker running amok or whatever). Had once a phone call hang the device. But then: pre-production unit and firmware, abused by installing dozens of apps from extras-devel and hacking around endlessly. 2500 songs, 1000 pictures, 48 GB of memory :-)
d) If the browser gets slow, it can well be because of multiple flash containers in a page or similar hyperloaded webpages. On average news sites, I never ever had a problem.

Come on, nothing is perfect, but file bugs in bugzilla and help Nokia make this thing even better. For me, after 6 weeks of intense use, it's still the JESUS-Comcomcom (Compact communication computer) :-)