The Nook Wifi ships with a touchscreen and web browser. If you hack it you will have a rather cute little web tablet (with lots of apps). But even if you don't, you still have more annotation abilities than the Kobo ereader as well as a nice browsing experience.
The Kindle Wifi has a keyboard and browser as well as access to the largest ebookstore anywhere. I won't go over all its features (you probably know what they are.)
The Kobo Wifi lets you visit the Kobo ebookstore, but there's really no way to browse other sites (because you can't enter a URL). I'm surprised that Kobo didn't think to include a touch screen. That would have opened up a lot of possibilities.
When you line them up, it's quite clear that the Kobo Wifi can't match the abilities of the other 2 $139 ereaders. I really think Kobo should drop the price to $119.
What do you think?