As someone who made the same conversion:

1. NW/CO is different from US/UA. NW/CO is like two different brandings of the same airline. NW/DL is more like US/UA (where you basically ignore each other's elites for upgrades, but let them in the short line for check-in).

2. Check for CO flights as well. You can upgrade on those, though I think CHS is all regional jets for them, too.

3. It's not the US snack basket, though the meals are better if you're on a longer flight. (Also, the US seats are more comfortable, IMO.)

4. $80 each way. 3 max in cabin per plane, 1 in F and 2 in Y. (Significant restrictions on checked pets, but NW has the PriorityPet program which is supposed to be really good for the animals involved.) I did this just last week with a cabin pet in F, and saw several other people with pets. The link above has further details.

5. From WorldTraveler: Bacardi Rum, Bombay Original Gin, Dewar's White Label Scotch, Jack Daniel's Whiskey, Seagram's VO Canadian Whisky, Skyy Vodka, and Tanqueray Gin. Beer: Budweiser, Bud Light, James Page. Additional selection on international flights.

6. NW club membership gets you access to CO clubs and a few others as well. Also, NW Platinum gets you "SkyTeam club access" on exactly the same rules as a US CP gets "*A Club Access".

7. Yes, but they're not nearly as good as the US ones, at least for me. I don't see nearly as many routes on NW as I do on US, especially like European routes.

8. No...it's amazingly automatic compared to US. For the first few I was surprised how easy it worked...I never had to call in and harass anyone. After the upgrade, you can also go on the website and change your seat if you want, without ever having to talk to one of the agents.

9. Being just gold, I'll leave this for others, but remember you only have NW and CO plats to worry about.