as a racer:
Well marked course w/ proper distance markers or none. I can’t stand it when the mile markers are actually at .75, 2, 3.4, 4.2, 5.5, 6.6 for a 6.2 mile run. If your run is long or short let us know, we don’t care, we just want to know.
Lots of information from the starter. Solana Beach Tri has a very organized start and I wish more people would imitate them. 5, 1 minute, 30 sec, 10, 9, 8, 7, 6, 5, 4, 3, 2, 1. They also are very good about telling you what the next wave is.
Volunteers who know the course and are paying attention. Again Solana Beach has people directing at every place you have to make a decision of where to go. And the people are paying attention and yelling out simple effective directions. Some triathlons that I’ve been to have had people smoking and chatting with friends instead of paying attention and it has directly led to crashes on the bike when the people realize they are about to overshoot the turn.
Assigned or at least marked bike racks. There is nothing worse than arriving at your rack to find out that the people of either side have brought a cooler full of stuff and are taking up 2 times as much space as they should. I’m not a very confrontational person face to face and I hate having to ask the person setting up for a picnic in the transition to move their stuff.
I don’t care about finisher medals, and i would rather have tech shirts over t-shirts, even if they are target champion brand with no race logo. Just make sure that there is water being handed out near the chip removal. The first year the LV Marathon had no water until you exited the finisher area which included the line to get your photo taken with the show girls. So it took an average of 10 minutes to get water (which was in another tent 100m away) after you crossed the line.
Music, beer, and good food would make me want to hang out at the race site even if i didn’t expect to win an award, but it’s not me you need to convince, it’s my wife who has been standing around since 5am so she could see me 4 times during 2 hours of racing. Needless to say she’s almost always ready to hit the road once i’m finished. 
Security in the transition area.
As a spectator:
a course that has lots of places to watch, and maybe a packet that describes the good spots to view the race. For example the packet for Pumpkinman in Las Vegas should let people know that there are a couple of spots on the big climb into Boulder City that you can stop and cheer on your athlete and still drive to the top and cheer them on at the bike-run transition.
Lots of Bathrooms.