I get what you're saying, and half of it is a sign that you obviously can't enjoy Halo the way it's meant to be enjoyed. Nothing to do about that, though, I guess.
It's true that the Halo games have always been really bad at tapping into the amazingly deep story that surrounds them, but that's intentional design. It's an ACITON GAME for crying out loud. Yes, it's got some edges shaved off to appeal to a larger audience - that's why it's, like, the biggest FPS series in the world. I don't see how Halo is trying to be "badass" either. Badass is for Doom or Duke Nukem. Halo is deep and beautiful. The Chief himself is badass, but he's just one character in the middle of it all.
Do me a favor and play Halo 3 at least three times, and don't freaking rush it. Halo is MEANT to be a relatively slow game, for an FPS. That's why you have the insanely weak shields that recharge when you take cover. You're not meant to rush anything.
Everyone thinks Halo 3 is bland on their first playthrough. Most people play it again, because they know they'll like it more the second time - that's how the first two games worked as well. I didn't like Halo at all the first time I played it. I disliked Halo 2 the first couple of times. But now they stand out to me as some of the best games I've ever played. Halo 3 is the same. The second or third time through, the story will finally make sense. You'll start to find the seven hidden terminals (which each contain two seperate messages, the second set varies if you play on Legendary while reading them) which explain a huge deal of backstory. Every terminal I found made me go "Aaah!" at least once. If you want depth, you have to go deep to find it, but it's there. Also, you'll start to find new tactical opportunities that make every large battle twice as fun. For example, you know the first Scarab battle? I bet you took that one down by riding on a Mongoose and letting the marines shoot it with rockets, right? Instead, you could've stuck to high ground inside the buildings. They provide excellent cover and there are two missile pods there that let you take down the threats from a distance. Or, you could take the elevator up to the construction crane above. There's a sniper rifle there! Pick off the dudes on the Scarab from afar, then wait until it's close enough and board it by jumping onto it - no explosives necessary to destroy it that way. If you're feeling extremely gutsy, you could try to take it down externally by destroying the armor on the back, exposing the core to outside fire and taking it out without boarding it at all.
And on the last Scarab battle, you know which one I'm talking about, I managed to take out a Scarab by jumping onto it with a Warthog. Yep, there are ways to do that. Extremely quick - and extremely useful if you're playing the meta game and going for score, where time affects the end-of-level multiplier.
I haven't even bothered touching the multiplayer yet. Not even co-op. I'm still having too much fun with the campaign. On my fifth time through it now, and still finding new stuff on almost every level.