Yes, his ult needs to be reworked, whether that be reduce the CD, prolong the slow, change it to a stun, such as Veigar's Event Horizon. I honestly would rather have Event Horizon, than Death Sentence. Cast from a distance, 2 sec stun..
But his grab is ridiculous, if you play him right, you can lock someone down for 2-2.5 secs before they can move with an E follow up. Thresh's kit is fine. Just his ult CD is WAY too long.
Taric's ult is under a minute, AoE damage, and AD and AP boosts to all surrounding champs.
Lulu's ult is 80 secs, gives bonus health, knocks up nearby enemy champs, and slows them.
Thresh's ult, 160 secs, slows enemy champs IF they break a wall, then the CC after that first wall is useless, much less the damage.
I love his passive, I love his grab, his knock up is meh, but I can live with it, and lantern is very versatile. Ult just needs to be reworked IMO.