This comes up with us quite a bit, as we have a rather fluctuating and growing household. I live with my husband and my second life parter and another friend of ours. My husbands gf lives overseas but when she comes to australia, she lives with us. My husband and my partner and i have been living together for 3 years and have been together for around 7ish.
The most important thing for us has already been mentioned: space. a little place in the house you can call your own where you can get away to when you need to. Privacy is important, i want to be able to have quiet down time with each of them as well.
other then that, its all about time management =) and sharing household duties appropriatly >< all the boring household stuff that, if not tended to is one of the greatest sorces of household tension i have ever known, heh.