One thing I found interesting about the final polling averages was that they were much more correct about Biden's share of the vote, even when they were very wrong about the margin itself (since these polls don't add to 100%, that's possible). See below for the 538 polling average vs. the current NYT tally (% for Biden):

PA: 50.2% vs 49.7%

FL: 49.1% vs 47.8%

TX: 47.4% vs 46.3%

OH: 46.8% vs 45.2%

IA: 46.3% vs 44.9%

NC: 48.9% vs 48.6%

WI: 52.1% vs 49.5%

MI: 51.2% vs 50.5%

GA: 48.5% vs 49.3%

AZ: 48.7% vs 48.9%

NV: 49.7% vs 49.9%

Most are within ~1% or so (some for Biden, some for Trump) with some outliers being Wisconsin (-2.6%), Ohio (-1.6%) and Iowa (-1.4%) in Trump's favor still not being so far off (relative to margins of error).

I'm not sure what that "missing" bit in the polls really means (undecided?) but it seems the issue was that that bit of the electorate ended up going entirely for Trump in many places.

(edit: had the PA 538 # wrong)