You know there were a LOT of people that said, "Sure it had a good launch but talk to me again after a year and well see if it's still selling well then".
OK...here we are. Let's talk.
I personally don't think of this as a year (for me it's a year after the last core book comes out that's most relevant). But this is "A" point to talk about.
It's definitely great news for WotC and D&D in general. I know it's not been a big hit with existing roleplayers who I know, but it has got new (or very-long-lapsed) people into the game. That's been due to three things that I've seen (imho):
1) Perception that it's "new" and thus they can learn it/play themselves and without needing to learn a lot. This is of course a perception thing - all editions have been the same way, but it matters.
2) Really good marketing appealing to a younger generation of gamers, many of whom have heard of and are interested in pen & paper RPGs conceptually, but have never played them (but have played CRPGs etc. - particularly Skyrim and Dark Souls). Most 5E groups I know of are in their 20s and have no P&P experience.
3) Good rulebook design which gets people excited about stuff whilst minimizing the "I have to learn what?!" factor.
They could have done better on many things, but it's really solid for people who don't expect specific things.
Of course, there are more potential sales down the road - I mean, for example, the reasons I've not bought it, for example, remain the same, and could still be fixed by WotC:
A) No digital tools (and not even PDFs) together with a system complex enough to benefit them (unlike, say, Dungeon World). That's a dealbreaker for me.
B) Lack of content.