The answer seems pretty simple - expressionless eyes
The main reason why people hate him, find him boring or uncharismatic, is the fact he has very little expression in/around his eyes (not unlike Stephen Amell, who has an even bigger problem with this). That makes it pretty hard for him to portray believable emotion or to emotionally charge his lines and actions. Sure, there is some minimal range (anger, frustration), but not enough to consider him charismatic and diverse. There's also some issues with his voice/delivery (also pretty even and inexpressive at times), but if his facial expressions were on par, we'd probably let that slide.
Check his motivational speech in Clash of the Titans and compare it to some others (Russel Crowe in Gladiator, Mel Gibson in Braveheart etc.).