Molly and Davina from Marriage Equality California did this great visual
demonstration at every stop on our marriage equality caravan across Northern
California (always with Molly in her wedding dress and Davina in her royal
blue tuxedo). You can see the photos here:
http://groups.yahoo.com/group/lpsf-discuss/files/meca-dp.jpg
http://groups.yahoo.com/group/lpsf-discuss/files/meca-cu.jpg
Anyway...
They had two clear plastic 2-quart jugs. One was full, nearly to the top,
with candy (it was Valentine's Day, so the candy was those little sugary
hearts with the "Luv U", "Be Mine", and such printed on them). The other
one had just about a dozen candies, not even enough to cover the bottom of
the jug. The full jug, with approximately 1500 candies in it, represented
"marriage" as George Bush defines it -- 1100 federal rights plus several
hundred more rights (depending on each state and local government). The
nearly empty jug represented domestic partnership rights, as currently
offered in California and a handful of other states.
Then they pulled out another smaller container (maybe a couple of pints),
completely full of candies. This container represented the most recent
Vermont "civil union" or AB205-style California "domestic partnership". It
looked impressive in the small container -- until they poured it into the
nearly empty container. Then, you could see that it still wasn't even a
quarter as many rights as the full "marriage" container.
I think this was the absolute best and most obvious repudiation of the
"domestic partnerships and civil unions are enough" argument.
Anyway, since I clearly don't have time to list all 1500 rights that
straight married couples currently have, or even the 200 or so rights that
are currently offered in Vermont and will be offered in California within
the year, thanks to AB205, let me suggest that you go and get a copy of
Davina Kotulski's new book, "Why You Should Give a Damn About Gay Marriage".
She has compiled and printed an exhaustive list of exactly what you're
asking for, Mike, on "how non-traditional couples' rights are
improved by 'marriage' rather than 'domestic partner' status." Hope that
helps.
Thanks, everyone, for being so supportive. 
Rob