I really like ET. I consider it, Peter Pan and Spaceship Earth to be the best dark rides in Orlando. The secret sauce is the extra 3D element of going up and down, flying through space, etc. Because of their flat layout, other dark rides like Pooh and Cat in the Hat just can't compete. (I would however put interactive rides like Buzz Ranger and MiB in a different category.)
Peter Pan has cheap-looking painted sets and figures, but there is one thing it has that really beats ET - the moving headlights on the cars! I heard on a podcast that those are nothing but dots of glowing paint on bicycle chains. Cool
Other posters have raised all the points I could think of about ET - the preshow is boring, the inside queue is the greatest, the audioanimatronics (or mannequins) need work, ET's planet is bizarre, like visiting a department store Christmas display on acid (er, I mean, like what I *imagine* that would be like

), and ET hardly ever seems to get anyone's names right.
If I had a couple of mil and was told to fix the ride, I would redesign ET's planet. It should be more earthy and homey feeling, like the forest scene at the beginning but with alien touches. That would explain why ET was so happy visiting earth and collecting plants, because it reminded him of his home. The cedar/spruce smell of the earth forest could be replaced on ET's planet with something a little more exotic, like sandalwood or some other far eastern or african wood smell, which to the average North American nose would be alien and exotic. Some unusual, exotic but real-looking plants could be added, not too freaky like the Little Shop of Horrors, just unusual. Sort of like Endor without the teddy bears. George Lucas may be frightful at coming up with offensive stereotype aliens, but he's absolutely a genius at designing believable alien planets.