Midi-chlorians are the biggest single mistep
George Lucas made in
The Phantom Menace (though there are plenty of others that are nearly as large), or for that matter in the entire
Star Wars Saga. In one fell swoop, Lucas manages to drop his
Star Wars frachise from a
magical experience to the run-of-the-mill formulamatic sci-fi that is
Star Trek.
An unexplained phenomenon (The Force, or the cause of the crew's consternation in that week's episode of Star Trek) is neatly solved / explained away by a fantasy particle (Midi-Chlorians, tachyons, tetrions, subspace particles, what's left of Picard's hair etc.).
What Lucas evidently failed to realise was that The Force was best left a mystery - it was mystic which was all it needed to be and attempting to explain it or account for it ruined it. Mercifully, Lucas pulled off a major league Jedi Mind Trick by not mentioning this enormous mistake in Attack of the Clones. Having not seen Phantom Menace for three years and with the magic of Clones filling the screen, I managed to forget about the midi-chlorian debacle the entire time I was watching clones. However, looking back on The Phantom Menace, I see that I am not that weak minded, as neither Lucas or his magic digital movie that is Attack ofthe Clones can make me forget what he did.