Background: I came from a Treo 650, which I had a lot of open source software for, but was buggy and slow. I played movies on it, read ebooks on it, sshed, browsed the web, played NES and GB games, kept passwords, kept track of my mileage, etc; most everything you can do with an N900, but slower and not nearly as stable. Still, I've been very WOW'ed by:

• Streaming Internet radio around half the world and playing it in my car, without even an audio patch cable.
• Watching YouTube videos
• Having multiple web pages open and being able to switch between them quickly.
• Opening up a terminal and typing "cat /proc/cpuinfo" and "uname -a" and "perl -e ...", etc, etc.
• Finding equivalents to almost all the apps I used on my Treo already available for free.
• Keeping a copy of my favorite movies, music, ebooks AND the entirety of my local big band's sheet music (approx. 12k pages, approx. 300 pieces with approx 18 parts each piece) on one device (the storage on this device is incredible).
• Running Final Fantasy VII in not one, but two PlayStation emulators.

That may not seem like much, but keep in mind my standards are pretty high. All of the things I listed the Treo as doing, the N900 does and does better, so you could count those as "WOW" moments as well.

EDIT: 2010-04-07:
A few more that spring to mind:

• Exporting my N900 desktop over VNC to my desktop computer.
• Mounting NFS off my file server at home to play all the myriad media I have on it.
• Plugging in my N900 to my TV and seeing fullscreen playback work beautifully,

Installing Media Box, and discovering my N900 works with PS3 Media server, awesome.

My latest Wow moment came from playing GLTron on the N900 without any slowdowns whatsoever. B)

I've just been watching the UK Election debate live on the n900 - who needs a tv.

Learning about obscure settings while overclocking. Fun!

Being able to file my last minute taxes online from my N900. I'm such a friggin procrastinator.

Imagine you are in an oil platform in the middle of the sea. Only thing you can see is water and other platforms (Prove: http://www.facebook.com/photo.php?pi...5&id=123201225). You came as a favor to somebody in your team, although it wasn't your responsibility. You're gonna spend the weekend there, but operations you came to do are delayed.
You got an old laptop with a damaged sound card (Did I say old?), only one internet port that you have to share and is so slow that borders in the useless. You don't have admin rights, so installing games (Even flash player!) is not an option.
Only one TV in the platform and it is always taken by the crew to watch things you would never watch. Even the fitness room is useless; a/c is damaged, so you're running in a sauna (Not recommended, believe me). You couldn't bring any book 'cause your bag was already full.
What would you do? Jump off the board? Hah! I've got a N900!!! I can still watch movies, play games, read books, watch TV and even surf the web!!!
Isn't this a wow moment for me???

Watching 'Spaceballs - State Of The Art' full speed and stereo sound got me greening.

Talking to MohammadAG whilst he turned independent lights on and managed to create this effect:
http://www.youtube.com/watch?v=g3pvdFpA1II

My true WOW moment was yesterday ( I got my N900 just a few days back) when I configured my N900 to connect via ash alias and server key authentication to every of my 4 dedicated servers and thus for the first time have the freedom to enjoy life, to travel, eat ice cream or have a cup of coffee or enjoy the beautiful beaches and islands in my host country (Philippine islands)
while staying connected to my servers and KNOW that all is OK. this mobile server connectivity adds very substantially to my peace of mind!
I have been waiting for a true Linux mobile since some 8 or so years now. As a 12 years Linux user (opensuse) I love a true Linux mobile!!