If only elementary column was shown, I wouldn't guess what the blue dot means. It's unread message, right? Envelope with star is logical. Envelope means letter and star in this case means new. (Like *01.01.2012 †02.01.2012 means "born 01.01.2012, died 02.01.2012".)
Second row works for all 'distros'. First and third one works only with elementary. What I mean is, the highlighted area with the gradient star doesn't look good, the value transition makes part of it blend with the background and seems just a bit... crass.
If I had to choose something that worked well for everyone, it would be the symbolic icons.
elementary column looks best out of all of them, of course.
I think that the colour icons look better than the symbolic icons.
New messages should have bold titles, and read messages should not have bold titles to add emphasis on what's been read and what hasn't been read.
That's just how I would have it, but either way, it still beats the other two options (even though my favourite colour is orange...maybe if Ubuntu had a better colour orange...). Out of the current options, I would pick the Bold Titles and Colour icons.
bold really differentiates title, from participants, from text. i think it helps to identify everything really fast and get to reading.
symbolic icons don't look very coherent with almost every other ubuntu/gnome/elementary application. color icons also look good because they differ from bold text color, otherwise it looks a bit cluttered.