Alexrd said:

But I still don't see how does it make the Force less mystical, when we are only talking about the link between a being and the Force, not the Force itself.

It makes the link itself less ambiguous and mysterious, which was part of the mysticism in the OT.

We were never sure how someone was able to manipulate the Force - it was implied that people with certain ancestries had more of an aptitude toward Force ability than others ("the Force is strong in this one" clearly implies that Luke has more of a natural Force aptitude than, say, Wedge), but it was never stated outright that only those whose ancestors were Force-users could become Force-users themselves. There was an implication that some were luckier than others, but those who weren't blessed with the "gift" of the Force could still use it through hard work and dedication, which is why groups like the Jedi and Sith formed (at least, that's what I thought before 1999).

Making a person's connection to the Force tangible and measurable in any way completely destroys this view, meaning that only a certain select few could ever use the Force, no matter how hard they worked or how dedicated they were. That is my problem with midi-chlorians, and why I think they destroy the mysticism of the Force.