Ohh... the contests. The good, old contests. How I miss them from the newer games.
They were so... awesome in their simplicity, brilliant in their enjoyability.
I guess you could start with the locations. Because they were placed in different cities all over Hoenn, it gave you an incentive to walk around and visit lots of places in the region multiple times. It made Hoenn feel like a place where things happened, there were things to do in every town. You had an excuse to return to every place, much unlike the towns and cities in D/P and B/W. The contests made Hoenn alive. Especially Lilycove bloomed with the Master Rank hall. You heard about Lilycove long before you got there. It was a place you looked forward to see. And when you saw it, it didn't disappoint. A shame all the contests were relocated there in Emerald, really.
It also made every Pokémon usable for something. You could feed your trusty Dunsparce with PokéBlocks, teach it some nice moves, and it could ace the Contests and rule Hoenn despite utterly sucking in battle. Pokémon such as Sunflora and Seaking could get their time in the limelight in the contests. It gave you something to do with the Pokémon that couldn't be used elsewhere.
PokéBlock making. Oh, how fun, yet challenging, that microgame was. Gather a few friends, and you could make really awesome blocks needed to evolve that ugly fish you found at Route 120. The concept was also simple: Make PokéBlock, give PokéBlock to Pokémon, score lots of points in contest. No need for silly accessories or wacky dancing. Make, feed, win. Easy as.
Then the simple yet intriguing mechanics behind it. I won't go into details here, but oh how I prefer them to the multiple judges and stuff and stuff added in the "Super Contests".
The music. Fanfare!
Then, the paintings. The only thing I missed about those was the ability to take them with you and put in your Secret Base. Still, the sidequest of filling the Lilycove Museum with Master paintings promised some interesting stuff to decorate your base with.
All in all, I LOVED the R/S contests. I hope GameFreak leave them as untouched as possible if they ever remake R/S.