Would you play the same music for a pool party as you would for goth-inspired Halloween party? I'd advise against it.
Music is the wallpaper that can hold your mood together or make it fall apart. I'm not talking about theme, here. Themes are easy (tropical, Vegas, 70s) but mood is a little more elusive. For instance, let's say you're throwing a daytime pool party for about 35 people with a median age of 30. Just adults - no kids. You want the mood to be light, fun, and perhaps a little glam.
The mood for aforementioned Gothic Halloween, on the other hand, is dark and sexy, with a touch of evil. If the mood is right, the guests won't notice. On the other hand, if Katrina and the Waves comes on in a room full of red light, spidewebs, and black candles, there's a disconnect and everyone will nervously titter and say "Well, that's certainly unexpected..."
Another thing to keep in mind - party energy. Start out with some slower, lower energy songs. Keep the kickin-est tunes for about 3 hours into your party (so, 11pm for a party that started at 8pm)
Here's a sampling from playlists I've created for various events...
Gothic Halloween (dark, sexy, evil. Mix of rock and electronica)
Hey Pretty - Poe
Machinehead - Bush
Faint - Linkin Park
Devil Inside - INXS
The Bad Touch - Bloodhound Gang
How Soon is Now? - LoveSpitLove
Black Velveteen - Lenny Kravitz
Frank Sinatra - Miss Kittin
Impressive Instant - Madonna
Medazzaland - Duran Duran
Mega C - Fischerspooner
Robot Rock - Daft Punk
Big Dinner for Family and Friends (Elegant. Mix of jazz/big band and chill-out electronica)
From this Moment On - Diana Krall
Spooky - Dusty Springfield
Eres mi Vida - Mocha Lab
Desafinado - Stan Getz
I Concentrate on You - Frank Sinatra
The Gardens of Sampson & Beasley - Pink Martini
The Cure and the Cause (ambient mix) - Fish Go Deep
Dance me to the end of Love - Madeleine Peyroux
Huge Rooftop Party (Whatever the kids are listening to, plus some extras)
Viva La Vida - Coldplay
Lonnie's Secret - Owusu & Hannibal
Sexy Can I - Ray J
No Substitute Love - Estelle
Time Won't Wait - Jamiroquai
Love Today - Mika
Forever - Chris Brown
Don't Stop the Music - Rihanna
D.A.N.C.E. - Justice
4 Minutes - JT & Madonna
Good Life - Kanye West
Dangerous - Kardinal Offishall
Feel free to use these as jumping point for your next party! Most are available on iTunes. Speaking of iTunes, there are some great playlist suggestions on there if you're stuck. Spend an hour or two exploring their ideas and mix them with your own existing tracks.