I'd rank guest list as the single most important factor in a successful cocktail party. Has to be the right mix of people with similar, but not too similar, tastes. They should be really excited to be there and ready to shake it up a bit.
There are two way to build your guest list ...
The most logical and effective way is usually to network among your friends (or entities like social clubs) to meet a few really interesting folks who will, in turn, invite a few of their interesting friends. This will ensure a varied, but vetted, mix.
The second way is to buy your guests. Perhaps you run an ad on Hollywood Blvd inviting people to come. Or maybe you offer everyone who attends something free (and expensive). Either way, just make sure you're cool with the fact that many of the guests aren't there to really hang out, and probably won't ever come back. And also, expect to spend a lot of money in the process.
It's your call. There's merit in both, depending on your objectives. If want to throw the BEST party, option 1 makes sense. If your sights are set on outdoing the Jones family, option 2 could work.
After the guest list, the host is the next most important factor to a great affair. You can invite the most interesting (or just the most) people, but if you don't have a host who can get the conversation/dancing started, your party is doomed.
As the host, have you thought about your welcoming remarks? Have you poured over your guest list to determine what connections you might make? Have you thought about how you'll handle an unruly guest, or someone who comes expecting free stuff, and whatever you're offering isn't enough?
*if you haven't figured it out yet, this post has nothing to do with cocktail parties
Decor is tricky. You want to make sure the space is visually pleasing, but not overwhelming or distracting from the conversation. Enough cool things to look at and interact with (and for people to tell their friends about, so they come to the next event).
Not all of us are gifted in this area, but some of us refuse to acknowledge our limitations. I've been to plenty of otherwise nice parties that fall flat on the decor. Usually the problem is that the host is hellbent on pushing their style, rather than thinking about their guests.
I tend to include food and beverage in the decor bucket.
So what are you giving people to remember their great experience? What's that artifact that will sit on their coffee table or office shelf that will remind them of your great event? Even better, what will get their friends, family and co-workers chatting about the party, and jockeying for an invite to the next one? You have thought about this, right?