For what you describe, it sounds like the N900 will be your new best friend; there a few cons, that I'll cover in a sec, but for your requirements:

Music:
It is a fantastic device for music, with the onboard 32GB of storage, and the option to expand with an SD card upto 32GB, you wont be short of anything to listen to, with extra codecs you can play most audio files out there, including Flac.
I don't own an iPod but I'm pretty sure they can't play anything other than MP3/AAC.

The default music play is somewhat lacking in features, it does that job though, but there a better replacements out there, made by people from this community

Music playback can sometimes stutter, the only time I have noticed it is when browsing the web: you'll get a split second stutter every now and again, nothing too bad though.

Internet/Web:
The default browser is fast, easy to use and navigate and allows for pretty much a full blown web experience (including Flash 9).
But, you can replace it with others such as Opera.
I've used it the N900 make purchases with ease, and I'm always on forums.

YouTube works alright: it can be a bit fiddly to access the controls (I've only used the desktop version and not the mobile, so can't comment on that side); apart from that I have no issues with YouTube or other similar sites.
However, the N900 only supports Flash 9 right now, so sites which force you to use Flash 10 won't work.
(You can use a 'version faker' but I can't verify for it's effectiveness.)

Facebook and other IM services integrate perfectly, from what I've noticed at least.
You can chat to your online right within the SMS inbox, and all your online contacts are integrated with your normal phone contacts (pretty cool).

Some see the e-mail as the bullet in the head for the phone, I haven't noticed any serious issues though.
I only use gMail and Mail for Exchange, but it has been working ok for me.
When going online I can occasionally get a 'failed to update: invalid address' error but syncing manually and all is well.

Screen & build quality
Not much to say about it, but I love it.
Responsive, bright, vivid, sharp colours, great for watching movies on (if you can handle watching on a small screen).
Some will complain the it is not capacitive like the iPhone, others will see that as a relief (I personally do).

The build quality I'd say it good, no wobbly slider to my knowledge, though when sliding the keyboard in an out in it seems slightly at an angle, but when it's fully open it's perfectly fine.

Document editing:
There's many office programs for the N900, I don't know how productive you can be with a small screen and keyboard, but it's allowed me to do the odd bit of uni work when I'm on the train or lying in bed and can't be bothered to get my netbook out.

You can, with a bit of work and hacking, get external bluetooth keyboards to work.
As it's not an official feature from Nokia, instead work from the community it's not perfect (I have a few punctuation keys that I can't use, but then again I haven't really looked into it).

Other:
There aren't a ton of applications available for the phone, but all the main ones are there (mostly made by the fine developers in this community).

Ovi store lacks in badly in comparison to other other app stores out there, it's been no use to me, but has to others.
The mapping software is bogstandard: no great features, no turn-by-turn voice navigation (yet, at least), but for a bit of extra cash you can use 3rd party software.

Support from Nokia is questionable: you'll see many people and many posts that are unhappy, but for others it's not an issue.

Anyway, In all, it's an expensive device but has a lot but does lack in some minor (and a few major) features.

If you can get your hands on one to play around with few a few hours/days then definitely do it.
Often, you can find them for sale quite cheap 2nd hand on these forums and eBay (though be careful you don't get one of the fake Chinese knockoffs from there).