In general, I would really prefer people don't bring anything. I plan my party menus carefully and if I'm running short of anything, it's space to put things. My buffet table is laid out for aesthetics and traffic flow, and I select the serving dishes and utensils in keeping with the party's style.

More food arriving just makes things hard. It means I have to figure out where the hell to put it (messing up my table arrangement), and it means people don't eat the food I put my own effort/money into. And most of the time, they don't take their leftovers and/or dishes and I have to deal with the logistics of returning them. In fact, I currently still have other people's dishes on my kitchen counter, in the way, from 4 weeks ago. Ugly disposable containers in the midst of my white linen and crystal isn't much of an improvement.

Exception: If I *say* it's a potluck.

Exception: Guest has dietary restrictions and is bringing food within those restrictions.

If people feel they must bring something, I try to steer them toward non-perishable beverages, which can be used up gradually as needed or kept for the next party.

That said, when people bring things, I smile and thank them. They mean well.