I love it - but that theme is quite limiting. I had a fancy dress party for my 40th, which was
enthusiastically fulfilled by all those who attended (except one) - but maybe my mates are unusually pro-costume. The theme was easy-peasy though: "stage and screen", so if you wanted to go all out and hire an Admiral lord Nelson costume (
hendo) then you could - but equally you could come in your regular clothes and be someone from Eastenders or whatever.
I have two rules about fancy dress. I don't want to have to explain my costume, and I don't want to wear anything that makes me look less fabulous than I would do anyway... but I have a broad definition of fabulous.