Wow, good topic. I get a few excuses every year. I'm fine with people politely declining because as Magic said, it's not their thing, but bad excuses suck... pardon my French. My party is adults only and costumes mandatory, so some people don't come because of that.
Getting up on soapbox... The reason I do it though is because I'm not making the party all about the kids. It's ONE evening for adults to have a good time without having to chase their kids around the house and entertaining them. Costumes are mandatory because I've been to Halloween parties (before I had my own house and party) that fell apart because every subsequent year fewer and fewer people wore costumes until the last year she had the party, the ratio of costume to not was 1-to-3 respectively. It was uncomfortable for everyone and the next year she didn't have it. Despite my rigid costume and no kids policies, my party has grown from about 20 to over 60 last year.
With that said, the excuse I hate the most is... "well we got invited to another party the same night and kids are invited too, so we're going to go there." A major pet peeve of mine is how parents' lives these days revolve around their kids. When I was a kid and there was an adult party somewhere, my parents got someone to watch us kids and they went and had a great time! These days, 99.99% of parents revolve their entire lives around their kids. It's so unhealthy for everyone. Don't get me wrong, I think parents should spend lots of time with their kids, but not every waking moment needs to revolve around them.
Anyhow, my haunted decor is delicate and geared towards adults, not kids. It's not gory, but it's not pumpkins, apple bobbing, and kids in white sheets if you know what I mean.
My distant second most dreaded excuse is "I don't like to wear costumes", or "I can't think of anything to wear". Give me a BREAK! How brainless are you to not be able to come up with a simple costume. I could think of 100 ideas right now... of course not all would be the greatest. And the not wanting to wear a costume? Grow up... put a costume on and hang out with your closest friends and have a good time you dolt!
WHEW, that was an angry post. I'm sorry about that.

Sensative subject I guess.