For me, the following makes or breaks a show.
1) Friendly, reasonable vendors. Online, it's easy to be stubborn and rude, but when that transfers over in person, it can be a bit of a sour point on a show. Luckily it doesn't pop up too often, but there is always the one person bitter with the world who's scared of talking to real life humans. People who are outgoing, knowledgeable of the stuff in their booth, and who also seem to care more about the hobby than about getting top dollar for everything are the tops in my book. Plus, looking at other booths and vendors, not as competition (which so many people do), but as friends in the hobby.
2) Autograph guests. As a fan of hockey, I obviously love them, but as a sometime vendor as well, they also bring fans to the show who normally wouldn't go, and then they spend some money as well.
3) Browsing of selection. No I don't mean those booths where a person slams down a million toploaded cards into a pile and you have to go through it...*shudders*. No, I mean people with well organized boxes or binders sorted by team or player, it is an Indiana Jones esque adventure as you look for that hidden gem or card you've been looking for, for a lifetime. Or to find that one booth which sells actually game worn goalie memorabilia...*drool*.
4) Stuff to see. Sometimes I like to get my break from cards within the show, so I like to also look at stuff and learn about hockey some more. I model my museum exhibit to be as such every year. I sell stuff, I have several fun promotions, free cards for kids, and also my museum exhibit which gets countless compliments year in and year out. This year's idea is something special I have going...
5) Good conversations as mentioned by many before. I love talking, get me going and I never stop.
6) Actual attendees at the show. Even though I don't organize a show, I always love to see a huge crowd at a show. Why? Because I love seeing an event being a success. I love seeing kids, teenagers, adults, seniors, etc. there because it shows how the hobby goes through all generations and gives hope for its future.
I'm sure there's more I missed, but these always stand out to me.