This makes no sense to me. It's 100 seconds at level 1. That's actually a shorter cooldown than several bruiser ults(Jarvan, Renekton, Wukong, Garen...) It's short enough that if he fails to kill you with it but does enough damage to send you back, by the time you get back to lane it's almost up again.

He gets rewarded so much for using it correctly, so why isn't he punished at all for failing?

Good point, actually.

Hopefully it doesn't take you 100 seconds to get back to lane.

Well compared to Garen, Darius is actually not a good champion. If Darius had more sustain or mobility like Garen then I'd say sure, make his ult cd just as high. But Darius has no sustain, escapes, or even innate tankiness. I think his ult deserves to be better and on a shorter cooldown than Garen's because Garen has a much better kit for a tanky dps.

Renekton, WuKong and Jarven all have utility + damage in their Ults, hence a longer CD...
Can't say the same for Garen though, IMO, Garen's and Darius' should ahve equal CDs...
But yes, other bruiser's CDs are longer, and should be...

I find Daruis annoying. Think about,
Passive- Posion. Both teemo and twitch are built around this gimmick, and he has it and other annoying things?
Q- Punishes you from running, who thought that was a good idea?
The slow- So your giving armor pen to the guy that does ap, ad, and true damage?
The pull- so blitz's pull without the skill and can pull more people?
The ult- Need I get started?

He is a poorly built champion that is underpowered in lower-level play, annoying in middle-level play, and non-existant in pro-level as far as I know.

And to Certified, If is used properly, it shouldn't go on cooldown.

maybe because if his ult is ever down he might as well not be in the game. he doesn't have anything outside of his ult he can barely do anything else.

Darius innate ability in his ult, does more damage for targets with bleed stacks AND IT RESETS IF ITS A KILL. Yeah soooooo in team fights all he has to do is come in when everyone is half health, ult the weakest, kill, ult 2nd weakest, kill, ult 3rd and so on.

It's his reward for landing 5 hits in short succession as a mid-speed, clunky, lumbering melee only champion.