Via Polygon, here is a lot of great data about Steam via Steam Spy.
First of all, Steam is only 15% of the market? If the number was 50% I would still think that’s a bit low, but 15%? What is wrong with you people, catch up already! Of course, major titles like LoL or WoW aren’t on Steam, so that 15% number is a little misleading. I bet if you took out the top, say, 20 titles, and then redid the percentage, Steam would be a much bigger factor.
Second, GTA:V and Fallout 4 dominated the year in terms of revenue and copies sold, which also isn’t a surprise, even though F4 has only been out for a few months and hasn’t seen a significant discount yet. It was nice to see indie titles like ARK, Pillars of Eternity, Cities: Skylines, and Darkest Dungeon make the top sales chart. Everyone who bought H1Z1 to put that drek on the chart though? Shame. Shame. Shame.
Hardcore Casual 
How big is the whole market? Steam hit a record of 12 million accounts logged in over the New Years weekend. That sounds like a big number. Is it?
If we’re working with that 15% number, which is pure extrapolation based on sales figures, what is that in actual number of players that represents? As a percentage of the video game playing population, I’d say that Steam might be over represented at 15%, because it is a place that caters to more hardcore players that buy more games.
Basically, not enough data.
12m concurrent, which is a huge number, though still less than 10% of the 130m+ total active accounts Steam has. But that’s different than what the 15% is, which is sales based on the estimated total PC gamer market.
I think it would be really interesting to see what % of sales Steam makes for a game like ARK or Darkest Dungeon, because I have a hard time seeing it lower than 50%, and wouldn’t be surprised if it’s as high as 80%. For such games, Steam is huge. For a AAA title like F4, it’s important (as Bethesda themselves said), but still just a slice of the overall pie.
Over half the games I buy are bought from Steam, yet as a revenue percentage the amount of the gaming dollars I spend on Steam is very small, because A. games on Steam that I buy are much cheaper than AAA titles that aren’t on Steam. and B. subscription games are more expensive over time than anything else. Steam is Amazon, crushing everyone when it comes to the long tail, but games with monthly subscriptions are like utility bills. No one holds it against Amazon for not dominating the utilities market.
I have no idea how reliable newzoo is. I can’t quickly fond the article they refer too, but I did find one that states a number for belgian amount of gamers. Comparing that to census data implies that over 90% of belgians between 15 and 54 are gamers. It is hard to say how correct the 15% is withouthaving the data.
I’d be curious to see the average sell price by unit for each game. There’s probably a story to tell about which games were huge hit, ie mostly bought at full price, or another one about the lifespan of games. The economist in me is getting way too excited about this kind of aggregated data. ;)
I think that’s possible on Steam Spy, as it tracks not only sales, but what the price of a game is during the sale. In the linked post, they do have a column for max discount, so for instance W3 in Nov was 50% off, as was CS:GO just above it. CS:GO sold 5x more copies, but due to the price difference, revenue was close to even between the two.
I’ve read that LoL has more concurrent users than the top 100 Steam games combined, that blew me away.
I wouldnt take LOL for an example of anything, for it is just an outlier just like WoW was for a few years.