My oldest DS was 3½ for his first trip. He's also tall; he met the 40" requirements. He's a HUGE train fan and was positively insistent that his first ride by Big Thunder Mtn Railroad no matter how much I tried to coax him onto something more low key first. We walked over there and watched several rides. I pointed out that it went fast, that people were screaming etc. He didn't care, he was ready to go! He did great on the ride but when it was over he was very ready to get off.

He said it was a little too fast for him. We didn't try anything else "big" on that trip. We went again just before he turned 4 and though he wouldn't get on BTMR again he wanted to ride Splash Mtn. He was fine on the big drop but totally lost it over the 2 smaller drops in the dark. We had been preparing him for the big drop and didn't even think to mention the others. If they hadn't been in the dark he'd have loved it but since they were, he was in tears. He also thought Goofy's Barnstormer was too fast on that trip. My 2 year old however LOVED that ride and rode it about 3 times in a row giggling and grining the whole way. We carried him kicking and screaming away from it b/c he wasn't ready to leave.
We had good luck with our boys by just letting them know if there was something they should expect. If it's dark, goes fast or makes a loud noise we tell them in advance and let them decide. The CMs that greet you are great about telling the kids what to expect. We bring penlight flashlights they can have with them on dark rides but they have instructions to only shine the light on their shoes or the floor of the vehicle so as not to disturb other guests. The favorites for my then 3 year old were:
In the MK:
Jungle Cruise
Peter Pan
Dumbo
IASW
Pirates (made sure to warn him about it's drop in the dark!

)
Teacups
Tomorrowland Transit Authority
Astro Orbiter
At AK:
Safari
Kali River Rapids
Triceretops Spin (like Dumbo)
the Bone Yard
the train to the affection station (it could be a train to nowhere and it would rank among his favorites though!)
At MGM:
meeting Buzz and Woody and that is absolutely it
At Epcot:
the river ride in the Mexico pavillion
Journey into the Imagination
Living Seas
Universe of Energy