#1 Poor mic skills
Braun is very poor on the mic. His promos bore me, and that's because he just doesn't understand what voice modulation is. He holds the mic very close to his lips, and we can literally hear his voice dispersion.
His expressions more often than not remain the same irrespective of the circumstances. This makes his promos even more uninteresting. Place Braun Strowman against Dean Ambrose or Seth Rollins, and you'll see who gets more cheers.
Braun was heavily cheered earlier this year only because the majority of the WWE Universe hates Roman Reigns for some unknown reasons. For the way he portrays himself, he would be a perfect fit for a tag team, but as far as being a singles performer, he is clearly overrated.