So when I upgrade the blog I've just done, I'll be able to report back with more details. But right now, the big news seems to be a theme customiser. How this will work with custom themes - which all of our WordPress projects are so the design is unique to you - I'm yet to see but it looks good if you just want a bit of freedom over your default WordPress themes.
I think the fact that you can customise the height and width of your theme header image will let people with a default template have more freedom over what they do there. And I'm looking forward to just copying and pasting the URL of a tweet for it to be neatly embedded on the page!
And incase you're wondering, it's 3.4 "green" in honour of Grant Green - a jazz guitarist from the 1960s and 1970s who Matt Mullenweg and others from the WordPress crowd apparently listened to a lot whilst working on this version.