Lately his comments seem to have fairly consistently been on the theme of low returns. The pension angle is a bit interesting because pension funding obligations for companies with underperfoming pensions should feedback as an additional drag on the market.
For what it's worth, a decade of 4% stock returns wouldn't really surprise or deeply worry me. It does seem like we're at pretty high valuations right now,, and 4% is hardly worst 10-year return the S&P 500 has endured (-3% nominal, -6% real). If this site is right, historically somewhere around 15% of 10-year periods saw worse than 4% nominal returns.
That's the kind of thing we have to be prepared for, both mentally, so we don't make unwise investment decisions chasing yield, and financially, so that we don't have so little margin in our savings that retirement runs beyond our target, or much worse, beyond our ability to keep working.
I understand why a lot of posters here disagree with what Jack has been saying lately, but some people seem even a bit sour about it. I'm not changing my plan based on his predictions, and not very confident that's what will actually happen, but consider this:
What happens if you listen to Jack, and he's wrong on this one? You overfunded your retirement. That's not exactly a devastating outcome.