I'm throwing this here just as a placeholder until I can find more time to update my FAQ and (still in-progress) Tanking guide to match. This info comes courtesy of poster KodiakX on the TestLive forums:

Once you reach enough Penetrate rating you reduce the chance that your attacks will be blocked down to such a negligible rating it should never happen. The DPS numbers from ACT collaborate this theory as with 0-200 Pen rating a Straw Dummy will block 13-15% of the time. At 300 Pen rating it will only block around 8-10% of the time. At 400 Pen rating you are down to 5% block. At 950 Pen rating you're not only at 0% blocks but you're also at a whopping 40% odd Pen chance with skills like Iron Maiden can easily push you into the 50% range entirely reliably (against a Straw Dummy).

Naturally as the difficulty of your encounters goes up so does the base Block %. For example tonight in Nightmares with 499 Pen Rating I was at a 5% block rate over all on average. This only furthers the problem described where you need ever increasing amounts of Penetrate and Hit Rating to mitigate factors like Glancing and Block.

If I was a betting man I'd wager that you need around 600 Pen and 600 Hit Rating to never get Glanced or Blocked. Eyeballing Glyph Kit amounts that'd mean out of roughly 1900 odd stat points (280 Head, 280 Weapon, 250 Major x3, 200 Minor x3 as rough estimates based on upgrades so far towards 10.4 glyphs) you could go 600 hit, 600 pen, and then have 700ish odd stats to play around with. You could dump that into Crit, but then you'd have no Crit Power to back it up. If you go with more Pen that'll leave you with about 300-400 of extra stats with nothing to do with them. Probably would put it into Crit still (roughly 20% crit total) enough to handle buffs/debuffs like in Darkness War's protection.

This still leads to the same invariable scenario where basically Crit is just garbage as a stat compared to Pen.