Aw you guys are the worst, it's the first day he's available! Give him some time.
Passive I agree needs a little buffing to feel significant. It's not good for locking down kills. But, if you're at an objective like an inhibitor or a nexus, congratulations, you just finished the job. And considering the rest of his kit is designed around zone control, he REALLY shines at securing objectives.
Q does not suck. It can be interrupted, and it does take a bit to wind up, but it's low CD, provides crazy amounts of CC, and has a HUGE range for a basic ability on a melee. Definitely not a bad ability.
His ult is difficult, but you can reactivate it to stop and ground slam, which helps make up for its difficult aiming a little. That's not perfect though. I also agree the yelling is obnoxious for strategic purposes, but taking that away might be like making any other semi-global ult not give away the fact that an enemy was teleporting, potentially right behind you. Because Sion's ult is, in some ways, a semi-global. I trust Riot put a lot of thought into whether or not Sion should announce his approach.
His kit makes TONS of sense, are you kidding me? He's an unkillable CC AOE tank! If you're building him around his passive you're doing it wrong. I don't even build AD on him; his shield is phenomenal, both in tanking potential and in damage (%health of enemy target, as an AOE, is nuts), and his Q has a great base ratio. He has lockdown with his E to keep the enemy in range of his Q and W pop, and his R is one of the best engage tools in the game; when used right, you can knockup an enemy team, pop your shield for %hp on all hit enemies, then charge your Q (while they're still stunned) and force them to either flash or die as your team arrives three seconds later. And he can do this to one, or MULTIPLE enemies simultaneously. How is that not amazing? Rammus is jealous of that. Sejuani is jealous of that.
Just give him a chance, okay?