brinked - 6:01 pm on Apr 20, 2012 (gmt 0)
I'd just like to add an observation re #4. I used to believe that too but around 10 months ago I had a site that received an oop. I changed several things and one was to move it to a dedicated server which hosts 2 or 3 other sites I run (same whois).
It's since recovered and is doing fairly well which leads me to believe that there isn't any x-site impact on an oop at the moment. That's not to say there won't be in the future though.
You can certainly get away with having 2 similar sites on the same server..so long as they are 2 completely different sites and offer something different.
What I have experienced, if you are running 2 sites that are using the same script and have the same/similar content, one of those sites will likely never see the light of day in google. You are more likely to get away with this if they are running on separate servers.
Keep in mind as well, I have not explored this in over 3 years as I keep any similar sites completely separated from each other.
I like to think AHEAD of google and anything that can be a factor will be a factor if it isn't already.