I don't like the Colosseum because it lacks in any sense of urgency or danger for Azeroth. I'm not some damn prize fighter, I'm a hero of the horde and I don't see why I should have to go into your enclosed arena and kill some worm unless said worm is about to unleash the curse of a thousand boils onto the entire population of Azeroth.
The interest of raids is having an important impetus to stop the actions of those inside the raid. Old gods threatening the planet. Scheming dragons manipulating whole populations. Crazed Kings who've made a pact with demons. There's none of that with the Colosseum.
Yeah very true. No big bad evil to kill here, just stuff the Argent Dawn rounded up and said
"Hey dudes fight this shit to prove your worth!"
"But. I killed half the black dragonflight, 2 old gods, multiple demonic leaders, Illidan and his underlings, destroyed Naxxramas TWICE and wiped out several troll civilisations!"
"Yeah... but can you defeat TWO Jormungar worms!?!"
Humor can be dissected as a frog can, but dies in the process.