Despite what people in the UK might think, Halloween is not primarily about dressing up. Moreover, it's not about dressing up like a slut or a zombie. I don't know where this came from (internal collective psychological trauma?) people in London seem to think that there are only two options for dressing up on Halloween: like a zombie or like a slut. On very rare occasions you'll get someone with some subtlety who goes for the slutty zombie look, but that's not very often. Given that the only difference between a normal Saturday night - because another thing people here don't get is that Halloween is on the 31st, not on the last Saturday of October - and Halloween is that girls might smear a little fake blood on their faces as well as glitter. I digress.
If you consider that most Americans live in suburban planned communities and drive everywhere, interaction with neighbours is very limited. Also, unless you're downtown or taking your dog for a walk or going to the park, you never walk anywhere. When I was growing up, the most exciting thing about Halloween was that we spent all night walking around the neighbourhood filling up pillowcases with candy by going door to door. Families set up chairs and sat outside, people got out bbqs and had big street parties - for one night, everyone was outside and everyone was friendly. Sure getting dressed up was a big part of the fun, especially because my aunt always made us amazing costumes, but it was more about being able to run around the neighbourhood with everyone else and just have fun.
Like Christmas or Easter, Halloween in America has its own set of traditions that haven't made their way across the Atlantic. When you disassociate these traditions, then you have Halloween a la UK: fake blood, slutty zombie costumes and lots of alcohol. There's no pumpkin carving, bobbing for apples, trick-or-treating, spending a month making an amazing princess costume, or any of the things that really make Halloween so fun and so different. Halloween isn't about zombies, it's about community.