[mgmueller]

I'll add further features within the next few days...
As usual...just my personal opinion...

OS:
#1: BlackBerry PlayBook
Intuitive, straight forward, almost perfect.
#2: HP TouchPad
A bit less intuitive than PlayBook, but still exactly what a tablet should be about: No physical buttons, all touch gestures, highly efficient.
#3: Android
Lots of potential, 3.0 being a quantum leap from 2.2.
#4: iOS
Easy to use, very convenient. But extremely "simple".

Apps:
#1: iOS
Mass and often class.
#2: Android
Good enough, rapidly expanding. Getting closer to iOS.
.......
# 17: HP TouchPad
I wouldn't call it #3 yet. The gap is too big. Good quality, but way less apps.
# 2345: BlackBerry PlayBook
It's a shame. Such a brilliant unit, but close to no apps to use (I've got about 25, I simply didn't find any more half-way interesting apps).

I guess it's quite simple: Android grows exponentially because of various manufacturers and being the widest spread OS.
iOS would be way more exotic, but has the "cool" factor from Apple and is kind of the trendsetter.
PlayBook and TouchPad obviously suffer from being a single gadget. Hopefully they can bring some attraction to 3rd party developers. Or they simply go for multi-boot (Android + original OS). Or at least the Android emulator, promised for PlayBook.

Display:
Surprisingly huge differences.
#1: BlackBerry PlayBook
For whatever reason it's even readable in direct sunlight (well, what we call "sunlight" in Germany).
#2: iPad and some Android units.
Surprisingly, the differences in display quality are quite significant on the various Android units. Acer Iconia for example is "worse" than Motorola Xoom or Asus EeePad Transformer.
#3: Some Android units and HP TouchPad.
I've read disastrous reviews about Motorola Xoom. Quite frankly, I find it one of the better units. I'd rather call HP TouchPad a bit "washed out" than Motorola Xoom.

Multi-Tasking:
#1: BlackBerry PlayBook
Flawless and convenient, no noticeable lag.
#2: HP TouchPad
In some areas even more advanced than PlayBook. But sometimes it "stutters" for a second.
#3: Android
Not quite there yet, but already good enough.
#734: iOS
What multi-tasking?

Notifications:
#1: BlackBerry PlayBook and HP Touchpad
Almost perfect. All information there, but not intrusive at all.
#2: Android
Close to #1, slightly less appealing to me.
...
# 734: iOS
Disastrous. A notification pops up in front of my app. I'm in the middle of typing something. I'm not fast enough, keep typing and the notification is gone. Or it disturbs my ongoing activity.

Fullscreen of mobile phone apps:
It's not perfect on any of those units.
#1: Android and iOS
The standard mode of iOS is slightly better. At least you can double the size by simply pushing an onscreen button.
With "Spareparts" app on Android and jailbreaking iPad, both are similar. Not perfect, but way better than just doubling and pixelating the apps.
#734: BlackBerry PlayBook
It's tough to rank.
On one hand, all apps so far perfectly fit the display size. On the other hand, that's not too difficult for some dozens of apps.
#347680: HP TouchPad
So far, I haven't found any "hack".
The app is either fullscreen or it's not (LinkedIn).
I didn't even see any "warning" in the appstore.
And there doesn't seem to be a "doubling" option like on iPad.

Flash:
#1: BlackBerry PlayBook
Smooth and without need for any "hacks".
#2: HP TouchPad
A bit less fluent than #1.
#3: Android
Sometimes a bit slow and "stuttering", but working.
#4: iOS
What Flash? Only with 3rd party browsers. With these browsers, it's about even with Android.

Form Factor:
Personally, I prefer the 16:9 aspect ratio of Android to the 4:3 of iPad2.
I won't give a ranking here, it's way too subjective.
iPad2: 4:3
HP TouchPad: Almost identical to iPad2, I even can use the very same neopren sleeve.
PLayBook: 16:9
Xoom: 16:9

Cover:
There are tons of 3rd party covers of course.
For comparison, I use the standard one of the original manufacturer.
iPad2: It really is a smart cover. I love, that I easily can separate it. When reading, any cover only distracts and disturbs, so I simply put it aside.
But: The back isn't protected, so I use a separate body for that - fully smart-cover-compatible.
TouchPad: Almost a copy of the original cover of iPad1. Good enough and even fits better than the Apple cover did on iPad1. Still...feels and looks a bit dated...
PlayBook: I didn't buy the original cover yet. Only a back cover, which might be enough given the PlayBook's small size. Even this back cover probably wouldn't be necessary, as the back and the sides of PlayBook are rubberised already.
Xoom: I absolutely hate the original cover. It's stiff and huge. Well, it certainly would protect an entire Airbus...

Support:
Wow! On HP TouchPad, in the "Help" section, there's live chat with HP support incorporated. I'm just chatting with Veronica about Kindle app, GPS and such...


In general:
iPad and the Android tabs, in my opinion, definitely aren't the best tablets out there.
iPad to me has a single advantage: iTunes = tons of apps and multimedia content.
I prefer some of the Android tablets (Motorola, Asus) to iPad2. Some others (Acer, Galaxy Tab 7") are slightly behind iPad2.
BlackBerry PlayBook and HP TouchPad have way more advanced OS from a tablet perspective. PlayBook for now seriously lacks content, TouchPad seems to be on a better way.
Meaning:
If PlayBook can attract 3rd party developers or (quickly) offers an Android emulator or Android multi-boot, it can "kill them all".
If PlayBook keeps suffering re. number of apps, TouchPad may be the silent winner...