But if you believe that US stock markets are mean reverting around a long term value of Q ratio, or of PE10, then there are identifiable risky areas and less risky areas.
I agree. However the "long run" is very long indeed. By those measures the market has been at least 33% overvalued since 1992.
Just for kicks, here are the statistics regarding the ending balance (as of Dec 2009) of a $1MM portfolio on a 4% withdrawal rate for a retirement starting in each of those years (1992-2009).
Stock/Bond . . . .. 75/25% .. . . . .. 50/50% .. . . . . 25/75%
Mean . . . . . .. . 1,086,917 . . . . .. .1,037,101 . . . . .964,243
Median . . . . . . . . 854,526 . . . . . .. . 888,757 .. . . .909,818
Min . . . . . . .. . . . . 532,342 . . . .. . . .657,455 . . . . .774,381
Max . . . . . . .. .. 1,899,128 . . . . . . 1,561,032 . . . 1,215,082
Seems pretty balanced, notwithstanding that 10 of those years had valuations (26+ PE-10) at least 33% above current levels.