I'm not one to squabble over mismatches between source material and movies - as long as the story is consistent within its own universe.
From this point of view, this series has managed to put all of the recent Hollywood adaptations to an absolute shame.
First of all, the adaptation is so good that sometimes it's difficult to say where the Dumas novels end and where the adaptation begins. Sure enough, the events from the books aren't borrowed entirely and sequentially but rather they are somewhat transformed and scattered.
The main plot points are there, but with extra background and extended events which strengthen character building.
This is definitely a necessary step especially because the original story (so very popular) has turned the characters, their demeanor, names and status into stereotypes ... and stereotypes can be boring especially when the original context is transformed.
The musketeers keep their names, general demeanor but their background stories are enriched (while keeping the main points).
Aramis is a former priest with a soft spot for the ladies ... but there's much more in the hold. Athos remains somewhat misanthropic with lots of mystery behind him (and with a history alongside Milady de Winter) Porthos gets more of an overhaul especially given the original largely straightforward character type. D'Artagnan in turn seems to be more of a bon-vivant type now and although he keeps his original motivation and history, he is portrayed here a lot more child-like. I'm not saying this is necessarily bad, but we'll have to wait and see what the writers will make of it.
Richelieu gets a great portrayal from the future Doctor Who actor, greatly improving upon the original character with more details and more political plots which are mostly well-thought out as opposed to the incredibly brainless plots of the Hollywood movies. Let's hope they'll keep up the kind of work that we've been shown in the first few episodes.
Obviously, nothing is perfect. There are inconsistencies here and there as well as questions left hanging. But we've got at least a season to see where they lead.
