The only problem is that the feats that interact with Raise a Shield don't with the parry trait. I guess you can take Taunting Strike instead of Taunting Shield for effectively the same benefits, but you also lose access to feats like Defensive Advance or Reactive Shield.
I don't think all feats that affect shields should work with parry weapons though.
So... To rerail:
I really like that the Guardian has options for two-handed weapons by granting (or improving) the parry trait on them.
Otherwise, people would think you need to rely on shields and one-handed weapons to play a Guardian.
There is an awful lot of support for shields though, not having a shield means missing out on lots of feats.
A Captain America build almost seems optimal.
Trouble with shields is the Guardian lacks damage bonuses so relies on their weapon die more, especially if using Proud Nail (which uses an action to Taunt to set up). Plus you're spending an action until Paragon's Guard at 12th. That's a pretty tight action budget leaving little room to Stride, Stand, etc. All well if in a static fight in tight quarters, but how much fighting is that?
Of course if your Guardian is succeeding at drawing enemy fire, you're going to want that shield and perhaps accept that you rely on the offense of others which goes back to that X factor of party composition (as well as how dynamic fights get). There's a legitimate hard choice here, both styles tempting, so that's good.
As for the captain, I doubt I'd have enough Dex to be throwing a shield, though a free hand would be useful.