Not having anything to drink. If you are having people over at least buy a 2 lit. of something.
Yeah. I just come from a family where the second you walk in it's "can I get you something....Diet Coke, water, juice?" So to me, that's how you welcome someone.
When there are other guests there that you don't know and the host(ess) doesn't introduce you(obviously this applies to small gatherings, not large parties).
My XMIL was always shoving food at me and the kids, and her feelings would be hurt if we turned it down.
She would also turn the thermostat up to the melting-point-of-iron position whenever someone went to take a shower. I used to close the bathroom vent and pile towels on it just so I wouldn't melt.