Brislove, I almost lost you there.. If I read that right you are saying that the trade off the character chooses between dealing more damage with 2h weapons vs a little more protection with sword and board is enough of a balancing factor?
yep. Well that and the fact that they have to spend money on that shield. for an example:
2-handed half-orc fighter 10 28 point buy: According to the DMGs wealth by level (p135) we have 49000 gold.
Full-Plate +3 (~11k), Greatsword +3 (~18k), belt of str +4 (16k) Ring of Prot +1 (2k) some change. =~47000. Starting 20 str bumped twice.
Base Stats Str 22(26) Dex 12, con 16, others can be 8s, doesn't matter for this example.
Attack bonus is (21 total): 10 (bab) + 3(magic weapon) +8 STR=+21
Ac(27) is 10(base) + 5(level) +11 (full-plate) +1 (dex)=27
Damage pre-power attack is 2d6 (7avg) +3(MW) +12 (str) so 22 damage per swing. And hitting himself on a 6+.
Shield fighter 10 Same stats/race.
full-plate+3 (11k) shield +3 (9k) longsword +2 (8k) belt of str +4 (16k) ring of prot +1 (2k)
46k spent. but 1 point from the weapon had to change into a shield. That same change is a lot of items (10k gold worth).
AC(32)=10(base) +5 (level) +11 (Full-plate) +5 (hvy shield)+1 dex=32
Getting hit on a 12+by himself. Still perfectly reasonable, and would be a 7+ without the level based bonus.
Attack(20) is 10 (bab) +2 (MW) +8 (STR)=20
lets say no power attack, as it's not good with a 1-hander.
Damage(14.5) ~4.5 (weapon) + 8 Str +2 MW=14.5 so about 10 damage less per swing.
this discrepancy increases with higher levels as I'll show below. Again not to mention the money that has to be spent on the shield.
heres these 2 guys at 20 (720,000 gold so we'll just assume bunches of good things)
2-hander +5 greatsword +5 ring of prot, +8 belt of str, +5 amulet of natural armor, +4 dex item + 5 mithril full-plate. +5 insight to str.
AC(46) w/o shield is 10(base) +10 (level) + 13 (fullplate) +5 ring +5 Amulet +3 dex=46
Attack bonus(39)= +13 (str) +20 (bab) + 5(MW)= +39
So hitting on a 7 for ~31 before Power attack.
same guy with only a +4 weapon to pay for the shield.
Attack +38 hitting for 4.5 +13+4=~21
Same AC stuff +7 from shield= 53. so getting hit on a 14. With pretty twinked out AC.
if anything I think this rule-change makes it reasonable to use a shield. As without the +10 they would still be hitting you on a 4 making shields completely pointless 90% of the time (since the only time they make a difference is on a 2 or 3 since ones miss anyway.)
With just adding the extra AC you make shields a reasonable choice, instead of a way to make yourself do less damage for no noticeable benefit.
I guess my final review would be that the shield block is an interesting (and cool) option for a feat, but not a needed change for AC balance. it seems to be a trade of getting hit twice as often for hitting for twice as much. this doesn't change that low armored creatures get shredded to dust by power-attack and I think that is a favorable trade for not using a shield.