Kevin Drum tries to set Matt Yglesias straight, arguing that this logarithmic graph of real GDP demonstrates that growth has not been any slower in the past thirty years than the previous thirty years:
What it mainly demonstrates instead is that it's hard to read changes in small growth rates on a log graph. If we instead go to the BEA data, and compute the CAGR (compound annual growth rate) over the thirty year period 1950-1980, it was 3.63%, while the 30 year period 1979-2009, it was 2.66% (2010 is not available yet). So the last thirty years are a whole percentage point lower growth. And that's despite the earlier period including the very rough years of the 1970s.
Another way to look at it is to compute the ten year CAGR for the decade prior to each year. That looks like this:
You can see that from 1956-1974, there was a period where every 10 year window in that timeframe averaged over 4% growth. Nothing like that has been seen since. And the last 10 years are the worst since WWII.