The end of August and most of September featured a bunch of games played, more or less doubling the number of 7th edition games we’ve had to this point. This will, generally, give us a much more solid set of numbers to work with when it comes to performance and matchups.
But first, let’s take a look at popularity. Going into NOVA, Space Marines were far and away the most popular army, with over 100 more games than the next most popular army, Eldar. This is even more significant when you consider the majority of armies didn’t even have 100 recorded games.

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In the last month, Eldar, Necrons, Tau, Daemons, Grey Knights and Space Wolves all had more games recorded and played than in the previous three. The Space Marines, while still the most popular, came down slightly in terms of percentage of the field, while the Eldar gained ground in terms of popularity. ­­
Other armies that saw significant rises in popularity were Daemons and Space Wolves. Space Wolves could be expected, since they recently got a new codex that has some exciting builds. We can also guess a rise in games for Daemons stems partly from the time needed to get a 7th edition Daemon summoning army together.
Meanwhile, the less popular armies largely saw a decline in popularity. This could be in part due to the competition level of NOVA, which made up the majority of the games in the last month or so.
Looking back to NOVA 2013, it’s interesting to see the way the landscape has changed, given a new rules set and six new codices (eight if you count Inquisition and Legion of the Damned, nine if you add Grey Knights―but that codex wasn’t used in NOVA 2014).

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From this quick chart, we can see that, while still popular, two of the more popular armies at NOVA 2013, Tau and Daemons, have taken a sharp downturn. Meanwhile, Space Marines, who were largely absent at NOVA 2013 (third least popular, above only Sisters of Battle and Orks) are now dominating the field thanks to their new book. In fact, the only armies that saw increased popularity and didn’t get a new book were Eldar and Grey Knights.
Moving back to the current meta, with NOVA and four months of 7th edition in the books, we have enough data to start looking at some matchup results (omitting any matchups with fewer than 10 games). First, let’s look at general Win% by matchup:

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Looking down each column shows an army’s win percentage by opponent. The top dog, Eldar, continue to have mostly good matchups, though they seem to have trouble with Knights, and Necrons have taken a slight lead.
Meanwhile, compared to  the end of 6th Edition, armies generally aren’t as overall dominant or ineffective. That is, while Tau’s only poor matchup in 6th was Eldar, with just barely under a 50% win rate, so far in 7th they seem to struggle mightily against Eldar and Daemons. Similarly, while Space Marines’ lowest win rate in 6th was against Daemons with 41.3%, they are now running into problems against Necrons, Eldar, and Grey Knights.
Finally, looking at Necrons, who were largely dominant in 6th, while still mostly winning their matchups, there are a few where they seem to struggle in 7th, like against Daemons, Grey Knights and Tau.
What this indicates is that the game has become more balanced in 7th, with some of the more dominant armies beginning to face potential counters. Eldar are still fairly safe bets, but struggles with Knights and Necrons, two armies that are popular and becoming more so, at least as an ally, could start to show some chinks in the current Kings of 40k.