After all the discussion on this forum about new Google Content Guidelines (no push ads, no home screen icons etc.) - we can now see the impact of that:
Number of available Android applications - AppBrain
New Android apps per month, October 7, 2013
(second graph at the bottom)
Unless this is a seasonal (or random variation) in the “new apps” numbers, there seems to be a very strong drop in Sept new apps numbers - they are 1/10 of August 2013. And the numbers so far for October suggest it will be 1/5 of August 2013.
It’s amusing to note that even though the total number of new apps has dropped greatly, the percentage of low-quality apps now vastly outweighs regular apps. (or at least it seems it’s going to, according to October’s graph)
So, the good app devs have given up, and the spammers are still going at it relentlessly? 
I guess it is either you spam via push / icon ads, or you spam by showing a higher impression 
Thus the higher number of low quality apps perhaps? (more apps = more impressions eh?)
In the post-Christmas/New Years period we can see that the number of app added to Google Play is at a much higher rate compared to before that period:
Number of available Android applications - AppBrain
While Sept, Oct, Nov new apps were low - as discussed on this forum - previously attributable to the Google crackdown on Airpush and other ad networks.
But in December we can see that new apps added have covered for all 3 months in that one month of December 2013.
In Jan 2014 - the app addition rate seems to be double what it was prior to the slump i.e. in June, July Aug 2013.