I'm not going to go back and quote a load of people, but to answer some of the thread:
I wouldn't say Spielberg puts aliens in all his films. I think this is about the 4th one he's directed that has included aliens. That's not many at all.
The original 3 Indy films were set in the 30's, and mirrored the cliff hanger type serials popular at the time. This is set in the 50's and mirrors the sci-fi men from mars type films popular at the time, so I thought entirely appropriate, and no more outlandish than the arc, stones or grail.
Lucas didn't take over the directing at any point. He wrote the story, David Koepp wrote the script and Spieldberg directed. Lucas probably did some second unit directing, which he also did on the other Indy films.
The musical motifs, snakes, line on the map were absolutely appropriate and should be in any Indy film.
When the fridge thing happened, my first thought was, 'that's fucking ridiculous'. However, then I remembered Last Crusade, in which an airplane gets it's wings knocked off as it hits a tunnel, then slides down said tunnel, the pilot able to make eye contact with the Jones boys as he does. Then the plane explodes at the tunnel mouth and the Jones boys drive through the explosion in an open top car and escape unscathed. That in no way ruined Last Crusade for me, despite being fucking ridiculous. From that point, I could fully enjoy the film and take it as it came. I don't watch Indy for realism, I watch it for high adventure. I don't believe for a second that a 900 year old 2 foot tall alien and lift objects with teh power of his mind, but I still love Star Wars.
As soon as I saw Area 51 on the guard house, I thought aliens were going to be in it. The corpse confirmed it, and then throughout the film there were suggestions, like the shape of the skull, the wall paintings. I wouldn't say it was sprung on the audience in the last 10 minutes.
I thought they did a good job of melding those ancient civilizations with aliens, a theory I've heard before.
In conclusion, arguing film opinion is like dancing about architecture.