One of the problems I'm having with it is there's no real way to confirm that it uploaded the pictures to begin with. I'll play around with it more but I reached my frustration point last night and gave up. I inadvertantly this morning figured out that if I right click on a jpg it gives an option to upload to iCloud...at least on my Mac. I had to do this on my PC as the requirements for Mac are OSX Lion...this is for the app that does the auto upload. I have Lion but if it's been upgraded to Lion, like what I've done, then they force you to buy an upgrade to iPhoto...didn't see that listed in their requirements to begin with. PC requires Vista or 7, interestingly there's a free app for PC.
When you set it up on the PC (assuming Mac as well) you can specify what folder you want to be your upload folder and it's supposed to automatically upload. I'm not convinced it was uploading and it may be a file size issue from some more reading I did this morning. Another thing I'm still a little uncertain about after doing some further reading on the subject is it sounds like the iCloud is a temporary swap space. They'll be there for like 30 days which gives your devices/computers time to download the photos and then they'll be removed as you upload newer photos. Doesn't really sound like what you want to happen and not sure how much the AppleTV will retain. I know it has like 8GB of memory but not sure how much of that is allocated for things like this.
Not many apps you are correct, that may be changing if you believe the rumor mill. Apple does typically do things more user friendly than others, but they also do things very closed source. Apple products play well, usually, amongst themselves.
I don't see any way you can just specify a folder for it to use....you can for a screensaver using iTunes but that doesn't correlate with the Photostream app.
No regular Hulu, maybe you can Airplay from an iPad/iPhone regular Hulu but I'm not sure. You could get a program like AirParrot to mirror your computer screen to the Apple TV but you may as well watch it on the computer screen then imo.
Not familiar with Twonky, Mezzmo but I'm guessing it's not for use with an ATV. Plex is what's usually used for ATV but it needs to be jailbroken which isn't possible yet for ATV3.