You can start with the fact that everyone knows where he's going. There's only so many places you can run to set up for a 3 pointer. Yet through a series of back-cuts, counter moves, running through screens, running, running, and more running, he somehow finds open space and is ready for the pass.
Then consider how quickly NBA defenders close out on 3 point shots. They know how deadly Ray is from outside so they're giving that extra effort to get out there in time, ...but they never really get there fast enough. Ray's release is so quick that it is a blur. Even better, his release is high, which makes it that much harder to block or alter. (His one bad shot Sunday night was the result of mis-timing the height and closing speed of Bynum)
Then there's the shot itself. His form is so good, to call it textbook would be like calling Mona Lisa a portrait. His balance is perfect (even when he has to lunge to the side to avoid a defender his upper body somehow contorts to a straight up and down form). His follow through is consistently consistent. The rotation is tight and the arch is... well, its beautiful.
The man himself is also a fantastic ambassador of the game. He's a nine time All Star but I get the feeling he's just as proud of his Ray of Hope Foundation and the multiple "Good Guy" and Sportsmanship awards he's won throughout his career. He is meticulous to a fault (borderline OCD) and as smooth as a politician when being interviewed. He even starred in a pretty good movie but didn't let acting or commercials become a distraction (see Shaq).
When he showed up in Boston, he took his All Star game and sacrificed field goal attempts and attention for the good of the team. A lot of people talked about how hard it was for Pierce and KG to sacrifice their games, but everyone pretty much assumed that Ray would be ok with it. Not because he doesn't have the mental makeup to be an alpha-dog. He was the star of Seattle for years. He was just the kind of guy that would do anything if he thought it would help the team win. He even learned how to play defense! Something that nobody gave him any credit for heading to Boston.
Yeah, there's a lot to love about Ray Allen. I'm just happy we got to enjoy him up close for the last few years. This is why it was smart not to trade him at the deadline. This is why I'd like to see him re-signed in the offseason. But all that is another article for another day. Right now I'm just enjoying watch Ray drain 3 pointer after 3 pointer in the NBA Finals.