Diamond Tiara, Prince Blueblood, Suri Polomare, the FlimFlam brothers, Garble and the dragon teens, and even Gilda(Whom I like for my own reason) all have personalities, but they stink(and that's an understatement).
Also we have characters like Shining Armor, Princcess Cadence, and Sombra, who(on their first appearance) had just as much, if not less character. And they still got a warmer reception to the series then Flash.
At least Flash isn't a jerk, he's a nice guy. At least he tries to help if given the chance. And he does seem to be actually supportive of Twilight(like when he mimicked her dancing so it would look less awkward)
I know he's generic, but that can easily be fixed if he was put in the show and given time to develop.
The only other complaint I've heard is because he's dating Twilight. And all I can say about that is: get over it. (not saying you complain about that)
That's just my thoughts.