It's funny that you were to say this. At this moment I'm at my office surrounded by 2 empty 1.5 liter bottles of water, and a 3rd one which is half-empty already, and thinking about going to buy another 6-pack.
People that don't know me often think that I drink a dangerous amount of water.

And no, I can't use my nails.

Well, I don't want to troll, but Android doesn't have multitasking. Randomly switching between the builtin RSS/Google reader app and the browser app sometimes causes one of the two to lose its state. Losing the browser app's state means reloading all tabs. A total bummer if I just went outside of the range of Wi-Fi with some selection of ~50 tabs that I was going to read.
If the browser app hangs (and I can make it hang by just tapping on a HTML5 video without a active connection) then I cannot switch out of the task until the Not responding prompt shows up (and it DOES take its time).
You cannot really open more than one window for an app. Sometimes I've resorted to using two IM apps in order to have two IM windows open...
You cannot really force an application to go back to the initial state (which can be done with close & relaunch on true multitasking platforms and a plain relaunch on WP). The Google guidelines now mandate an extraneous additional totally non-discoverable "back button" in the top-left corner of the UI (apart from the hardware one) to try and fix this issue, but I find it a poor replacement and is not even implement on all of Google's own apps.
It is not guaranteed for an application to simply put, continue doing what it is doing when it is backgrounded. Sometimes they do. Sometimes they do not. Sometimes they do but they do NOT appear in the "open apps" list which appears under settings (what does it exactly list?). No logic at all. Same when suspending.
The entire system is designed with this monotasking in mind (global back button, "activities" concept, "recent apps" instead of "open apps", etc.).

I hate it. Now, you might say that monotasking works for you. Which I understand, because I've been there, as a 10-year user of PalmOS devices (they were technically "multitasking" too). But if you come from a N900 background, you will face a _big_ time getting used to this crap, and I must warn that for some weeks you will keep on trying to do things that were taken for granted on the N900 or even on the N9.
If you get used to it -- perfect.
If you don't, then you will spend all the time cursing the damn thing all while thinking why should I force me through this clearly inferior solution (specially having already done ONCE the opposite transition -- from PalmOS to a Nokia Internet Tablet).

</rant>.