IMHO, the "main character personality" @El_Psy_Congroo is talking about is often one that is empty. It's the equivalent to the archetypical farmboy-turned-hero, and we've a.) seen it before so many times that we know how this goes, thus removing the suspense, and b.) when a character is too bland, it stops being relatable. There's nothing there to relate to. There's just a blank slate.
It's like trying to get emotionally engaged in a mannequin. It looks like a person, but it isn't one. It's just a collection of things.
I believe this is an entirely separate issue: you can have a main character who isn't a blank slate without it turning into one of these super-special-MCs (usually referred to as "Mary Sue", though I dislike the term since it's usually used to unfairly criticize female characters for stuff male characters get away with scot free). Special!MCs are bland for a different reason than the Main-Character-Syndrom!MCs are bland.
With the Special!MCs, there's no tension in the story. You don't worry about if they're going to make it or not, because of course they will; they're special. Anyone opposing them isn't someone who might have a fair point; they're evil and terrible (or jealous) because everyone must love Special!MC, because they're special.
There's a wide range between Main Character Blandness-Syndrome and Special!MCs, though. It's where characters with actual personalities, believable flaws, and actual character development live; you know, the kind of people who make mistakes, who learn and change, who aren't always right, and whose powers aren't acquired solely because the author liked the sound of it.
