Most parties I attend - and definitely all the best ones - are other people's parties. No formal invitations; casual invitations ensure that the relevant people find out and can decide for themselves whether or not to go.
For the most part, my list will have people who will be happy to be invited. However, there's no such thing as a perfect invitation list. There may be people who would like to be invited, and who I'd be happy to have in attendance, but who don't get an invitation, because I feel it's a bit of a stretch to say "we're close enough that you belong at my birthday party". And there may be people who receive an invitation, but find it odd, feeling that there's no real connection between us.
As I said to Dom a while back, I think of a friend as someone you can still relate to outside of the context in which you met them. Well, most of the people I relate well to, are people I only ever see in the original context. I feel welcome at Hartwell parties, but they generally consist of a group of people who actually are friends (by my definition). Where do I draw the line? And do I choose between people who don't get along, or invite them all?