1) It's unfamiliar, and this will slow people down. While this is not a major issue, compared to the rest, it may certainly increase support costs, which may not be allowable if you're working on Enterprise software with a huge userbase.
2) It would require a nonstandard UI widget in order to display the username in cleartext while hiding the password in the same field. If you're designing for the web, this means you're messing with people on mobile devices, which are a fast-growing segment of the population.
3) If space is the delimiter for username/password, it means that you can't use space in your username. I HATE when you can't use space in your username and possibly in your password, depending upon on how well-designed and well-followed your spec is and becomes. For one, I prefer having my username as "Dan Hulton", not "DanHulton", though that's a minor quibble. More importantly, more entropy is more better, and allowing people to use spaces in their usernames/passwords encourages the use of passphrases, which are LEAGUES better than passwords.
Also, having separate warnings for your username and password matches is a bad security practice. Best to just say "username/password combination not found" instead of letting a potential hacker know he's stumbled upon a valid username.