It seems there's been a morph into two seperate discussions, so I'll try to combine the two discussions into one.
The inital question was what makes a Disney ride "successful". Well, what does successful mean? Does that mean that a lot of people like it?
OR, do we mean it is a "Disney" ride? A ride that would make Walt proud?
Since I think that the original question was in reference to the latter, I'll answer that one.
A ride is successful if:
1.) it has a cohesive, identifiable storyline
2.) it fits into the area where it was built
3.) it fits into the grand scheme of the park in which it was built
4.) the show is carried out from beginning to end
5.) it is innovative (not necessarily new technology, but a new way of using perhaps an older technology or ride format)
For me these 5 criteria can fit anything that I consider a true Disney attraction.
I'll pick a ride from each park that I feel is Disney, and one that I feel isn't true Disney. (And everything is based on a park that is theoretically well-kept and maintained like it SHOULD be)
Magic Kingdom:
Splash Mountain is one of my favorite rides and one that I feel is a true "Disney" experience.
1.) The story line is very clear and apparent. At least it is to me.
2.) I believe that it fits into frontierland, even though marginally. I really think that this would have had a better home in Fantasyland. (I know that the logistics of that would have been nearly if not totally impossible)
3.) I don't think anybody doubts that it fits into the grand scheme of the Magic Kingdom. It belongs there! It's pure magic!
4.) Even on the parts where you could be bored out of your skull, they keep you entertained. Neat plants, and tons of things to look at. All of the effects (lighting, and sound, etc.) are well hidden.
5.) A ride that's been around forever: the log flume. Used in a totally new way.
Small World (I was going to go for the even easier one and use Aladdin, but I figured I'd not rehash that old discussion!

) misses the Disney mark, and Walt designed it!
1.) I can loosely feel a storyline, but that's not where I believe it misses the mark.
2.) I don't think this fits into Fantasyland at ALL. I don't think that there's anyplace that it would work anywhere in the Magic Kingdom.
3.) As I said in the last step, I think this ride is a sort of edu-tainment that belongs in World Showcase at Epcot. I don't believe it carries out the Magic Kingdom "feel".
4.) Lighting effects in full view. It looks like the Festival of Lights at Niagara Falls. Cheap, cheezy, and tacky.
5.) I do think that it was innovative. During a time of civil unrest and discrimination, Walt came up with a way of showing his love for all people. (If he was racist or prejudice, he did a damn good job of hiding it...I'm really not sure if that was his intent with this ride, but it's what I took away from it)
Epcot:
For a tried and true successful Epcot attraction, look no further than Spaceship Earth!
1.) The storyline? The development and progression of communication.
2.) One of the problems with labeling something "Future World" is it has to be ever-changing in order to keep up. But when the park was built, this was definitely in the future. One of the last scenes with the families using TVs to translate, and the one that translates? In the 80's that was just the most amazing thought!
3.) It totally fits into the theme of Future World. Because it tries to take a glimpse into the future.
4.) Definitely fits into Epcot. It is both entertaining and educational at the same time.
5.) As far as the 1980's were concerned, this was innovative. Nobody had known how to make a full sphere and not just a dome. Even now, the drainage system is too cool!
Something that misses the mark? Rio de Tiempo.
1.) While the history of Mexico appeared to be the storyline, it was somewhat loose.
2.) Yes, it definitely fits within the Mexican Pavilion.
3.) Yes, definitely fits into World Showcase.
4.) The show is carried out, very well I think. I LOVE the very beginning riding past the pyramid.
5.) THIS is where it misses the mark. There was NOTHING innovative about this ride at all. The boat had been done. No new special effects at all.
MGM:
The Tower of Terror. Whether or not you can get the courage to ride it is one thing, but if you doubt for a moment that it's a Disney ride, take a walk through the queue and use the chicken exit.
1.) Such an awesome storyline! DEFINITELY there, definitely clear.
2.) It sure enough fits into the Sunset Blvd. theme.
3.) Yup, it fits into the Hollywood park!!!
4.) The show is executed with such perfection. The attention to detail is absolutely amazing. As I said, just walk through the queue and use the chicken exit. It's AMAZING to see!!!!
5.) Definitely innovative. I don't think anybody has ridden an elevator that leaves the shaft, or that falls faster than gravity.
Missing the mark? The Muppets 3D.
1.) Was there even a storyline? If there was I don't remember it. In fact I remember one of the characters saying it wasn't a dumb excuse for 3D effects.
2.) Isn't this in the New York area of the park? Was the Muppet theatre supposed to be in New York? It just didn't fit to me.
3.) I don't see how it fits into the park. I don't think it had anything to do with the Hollywood theme.
4.)A great show!!!! It definitely hits the mark here for me. They actually destroy the theater!!!!!
5.) NOTHING innovative here. The 3D technology had been done in Epcot with Honey I Shrunk the audience. The same water and air tricks and whatnot had been used before?
Animal Kingdom:
Hitting the mark is the Kilimanjaro Safaris.
1.) Although it definitely felt tacked on, a storyline was very clear.
2.) DEFINITELY fits into Africa!
3.) DEFINITELY fits into Animal Kingdom.
4.) There is no better show than live animals as far as I'm concerned.
5.) I've been on open-air safaris before. We have one close to Buffalo in Canada. But they don't have climate controlled rocks, and lures to get the animals to come. They have trams, not unique modified trucks.
What doesn't hit the mark is Primeval Whirl.
1.) Storyline? Is there supposed to be one? If so I don't see it.
2.) It fits into it's area of a roadside carnival.
3.) It does NOT fit into the Animal Kingdom. Unless roadside carnivals now travel to exotic places.
4.) I don't think that there is any show to this piece. It just lacks. It's a wild mouse coaster. Nothing else.
5.) If the cars spinning on a wild mouse counts as innovative, then I suppose it is innovative. But it seems like such an antiquated ride.
Those are the factors that I think can show if a Disney ride is successful.