I may have missed it, but I haven't seen this diary mentioned specifically in any main blog or board post this year. I remember it well because it was surprising to me as well. All I can recall is Brian saying WRs usually don't put up big stats in their first year.
People seem to assume WR is like RB in that speed, size, and athleticism trump experience, but the Mathlete found in 2010 that returning WR experience is one of the biggest factors when predicting an offense's production. He doesn't mention TEs in his post, but obviously that's a big element in Harbaugh's offense as well. We went from having 2 senior WRs and a senior TE to NO seniors, 1 junior slot who missed a bunch of time,1 junior WR who has barely seen the field his entire career, and 1 junior TE who has strangely disappeared despite looking like the heir apparent. Sure, Speight isn't playing well, but we probably should have expected such an underwhelming passing game based on the losses of our receiving threats alone, at least at this point in the season anyway.
Here's what the Mathlete wrote:
This was the position that shocked me. I always considered the wide receiver position to be largely talent driven with little thought given to the value of experience for receivers.
In one of the strongest correlations I found, each decile of returning wide receiver experience was worth a half a point per game improvement. Even more surprising, the improvement wasn't restricted to the passing game. Of the half point improvement, only .3 ppg could be attributed to the passing game. Veteran wide receivers play a huge role in a team's progress. This may have been a fluky correlation for 2008-2009 but within the data set, it had one of the highest R squared values I found at 0.72.