I think that's mainly because it was aware enough that it was essentially a very silly movie to make it work. Somebody early on said "You know what this movie needs to be? James Bond in 17th-century France", and they just went for it.
It was horribly anachronistic, but I didn't mind because it was clearly not actually set in reality. I like Orlando Bloom as a villain a lot more than as an earnest heroic type. I'd say the main weakness was D'Artagnan, who needed to be about two or three times more charismatic for the audience (or me, at least) to actually root for him. Instead he kind of came off as a twerp. But back on the plus side, Ray Stevenson as Porthos: yes, please.
Basically, what I'm saying is, it's pretty much the fancy kind of boxed mac and cheese that comes with the pouch of cheese sauce instead of the orange powder. It's not high-class, and it's the product of a huge corporate machine, but it's cheesy and enjoyable and for what it is, it's pretty good.