Right, I upgraded my other blog which I also have this installed on.
After upgrading to 2.8 I checked a thread and it was still working fine. I then remembered that the plugin wasn't the current version as well so I upgraded that, then it did the same thing as my other blog.
So you lose the buttons at the top and the text area is just white with no border (fine on one but the other site has a white bg as well so you can't tell it is there unless you click and get a typing cursor)
As mentioned in the first post it displays the same in the preview area on the admin MCEComments Options page, so I didn't think it was theme related.
Now it has done the same thing on two different blogs using different themes it makes me think it is less likely to be the themes. If you want me to do any testing then I'll be happy to.
I also tried turning on the logged in users as well and as reported above it appears that the plugin "turns off" and you get the normal text box as if the plugin wasn't activated.