I know I'm probably restating an answer that has been said many times, but oh well.
Halo, when it was released, was a pretty exceptional game. While it wasn't revolutionary by any means, it was the best implementation of a console FPS to date (now usurped by Modern Warfare), and it altered the FPS formula in a few interesting ways that deviated from what we had grown to expect.
The problem is that for everything that Halo does to innovate, it keeps two other things stagnant. Halo: Reach is honestly not all that far removed from Halo 1, and that's not a good thing. Don't get me wrong, I appreciate that Bungie has taken their formula and refined it as much as they have, but the problem is that Halo hasn't changed (though I do thank them for getting rid of the horrendous Flood levels).
One of my biggest problems with Halo has always been its level design. Bungie has created a few gems, but there are some levels that are so badly designed that it's downright shocking (The Library, anyone?). The level design in all of the games has made me think of Halo as generic, even with the vehicles (which are endlessly entertaining, I'll grant you).
I tried playing Halo 1 again recently, and I couldn't even make it halfway through; everything in it was so dated that I honestly found it unplayable. Conversely, I played Half-Life the week after, and I was actually blown away. While the gameplay and art design in Halo was substantially better, the level design in Half-Life, a game from three years prior, blew Halo out of the water. When you have an older game like that completely destroy what has become the industry standard...you know you've got a problem.
Now, don't get me wrong. I love Halo; always have, always will. The art design and the music are phenomenal and the gameplay, while dated, has been polished to a mirror shine. Despite that, I have no qualms about saying that it is a generic shooter, and one that has been surpassed by titles that came before and after. But that does not make it a bad game.