The mystery of the Force helped to make Star Wars feel more classical, like a Greek epic, IMO. In ancient times, people didn't know the exact reasons or formulas for lots of natural phenomena. For example, they couldn't explain gravity; they just knew that things fell when dropped. Things like that were mysteries and just accepted as the way things were... and new things that weren't accepted yet were considered "magic." The fact that the Force was treated the same way helped Star Wars to feel like it really was taking place "a long time ago."
By introducing the explanation of the midichlorians and all of the political scheming, the prequels felt much more Age of Reason and Renaissance like (i.e. 500 years in Earth's past, rather than 3000 years). The classical epic feeling of the franchise was lost. Instead of there being mystery and clear distinction between good and evil, there was explanation and ambiguous allegiance. It's not that those are bad things. I want science and shades of grey in a lot of stories. It's just that I think that Star Wars is better when it's kept simple and classical.