As a 'cowboy' who has raised grass fat beef I have a few comments from my perspective and experience.

Grass fat beef produce more green house gases?
Maybe this is true but the time to slaughter in my experience is negligibly higher. According to the USDA 90.5% http://tokyo.usembassy.gov/pdfs/wwwf-bse20041112.pdf of the
cattle slaughtered in the USA are 15-20 months of age. All of my grass fed beef were slaughtered between 16 and 18 months.

Perhaps the animals produce more gas because of their diet but when you take away the tractor diesel fuel, fertilizer plants and all the petroleum products used to transport the grain used in feed lots I doubt that grass fed beef is less 'green' than feedlot fed beef.

Grass fed beef use more water than feed lot beef because their feed is dryer?
How is the water content of grass less than that of dried corn, dried wheat, and dried alfalfa?

Grass fed beef increases soil erosion?
I don't understand this one. Again the soil loss of tilling for grain production would account for many times the soil erosion of cattle.

Grass fed beef displaces threatened wildlife?
The land I see cattle raised on has been used for that purpose for over 100 years. If the wildlife was displaced the grass fed beef didn't do it. Lets talk about how much wildlife that new sub-division or the new shopping center displaces.

The gassiest cattle are feedlot cattle. Cows didn't evolve (or weren't designed, depending on your beliefs) to digest anything except tough, fresh natural grasses and shrubs. It's as if I were to go out into a field and start munching on sawgrass: I'd probably get diarrhea and tear up my insides with all the undigestible gunk. Same goes for cows. Cows are built to break down cellulose into simpler starches within their extremely complex digestive tracts.

When cows are fed a meal of oats, corn, dry alfalfa and bales of dry hay for long periods of time they develop chronic inflammation, which causes them, like it cause us, to waste away and become infected. Then the mass meat companies bring in industrial veterinarians to administer exogenous antibiotics and steroids to keep the cattle "heatlhy" and "beefy." All of which leads to a very unhappy cow.

Have you ever been to an open pasture cattle ranch? It smells a heck of a lot less than the feed operations that run along I-5 in California (and these are the small ones!). You can even see calves running around and frolicking in the fields, plenty of natural shade from trees, and there's usually abundant wildlife in the area as well. These are happy cows up until the very end, and that's a reality all of us have to face at some point.