That's true for Twitter, but not of Yammer, which is essentially a glorified mailing list. (Is there anything else that the Yammer model offers over email? If so, I'm not seeing it at cursory glance.)
Our company has over 200 employees on yammer out of about 2000 employees worldwide, so I'd say our adoption is somewhat large enough for me to comment on its dynamics.
Yes, I'd agree that it matches features with a mailing list but the types of messages that are posted on Yammer would be unacceptable to send as a company-wide email. People mostly post updates about where they are, conferences they are attending, and if they're working from home or remotely. Marketing-types tend to post links to the latest techcrunch or gigaom article. Designers tend to post the latest links off of delicious/popular. As far as the character limit, its treated just as twitter and no one posts anything too large.
With a corporate-wide mailing list, everyone would have to read every single message. People tend to correlate the number of recipients with its importance. Yammer messages can be easily ignored and read at leisure. As a poster, this naturally diminishes any inhibition when firing off a message. Perhaps it's just the obvious outcome of a messaging infrastructure wholly separate from our corporate email, which has expectations and political baggage. Maybe this says something about the state of email tools...
It could well offer fewer features, presented from a useful alternative perspective and without the old method's social baggage.
Having said that, my first instinct upon reading this feature list is to suspect that you're right. Yammer has too many of email's features. They need to leave more things out. Otherwise the pathological behaviors that characterize corporate email (poor composition, unclear expectations, inbox overload, endless chains of quotations, complex workflows, reliance on cut-and-paste versus publish-and-link, the use of Microsoft Office attachments) will just port right over, and nothing will change.
I'm guessing that removing the character limit will prove to be an especially foolish design decision: Who wants to read five-paragraph Tweets? And file attachments are probably even worse. One of the charms of Twitter is that it refuses to allow such dinosaurs. Instead it forces you to adopt an amazing modern technology -- the hyperlink -- to point people at your blog, or your Flickr set, or your Slideshare page, or your Scribd page, or your github account.
