I was having a discussion with a fellow clixer last week concerning the Guardians of the Galaxy ATA vs. Fortitude and I became very confused. Essentially, all I heard was "blah, blah, blah you are wrong" and "blah, blah, blah I am right." So I thought I'd take it straight to the source:

1.) What does the Guardians of the Galaxy ATA protect?

2.) What does the GotG ATA NOT protect?

3.) How does protection provided by the GotG ATA differ from the protection afforded by Fortitude?

1. Well, it protects powers from being Outwitted... just as the card says...
2. Abilities are not covered... so Outwit away.
3. GotG gives no protection against Exploit Weakness... other than that it's pretty much the same thing.

Well, you could just try putting them side by side and comparing them... but where is the fun in that

FORTITUDE
Cost: 25
Prerequisite:

Choose a character. The character ignores other characters’ Exploit Weakness. Outwit can’t be used by other characters to counter the character’s powers.


So Fortitude prevents Exploit Weakness from being effectively used against the character (though it could still be used to meet the prereqs for feats like And Stay Down!). And Outwit can't be used on that character's powers... and only powers. Abilities are not mentioned at all.

GUARDIANS OF THE GALAXY
Cost: 4 points per character on your force with the Guardians of the Galaxy keyword.
Prerequisite: Guardians of the Galaxy keyword.

All characters on your force with the Guardians of the Galaxy keyword are assigned this feat and have the following team ability instead of any other team ability:
Powers possessed by characters with this team ability can’t be countered. This team ability can’t be countered.

No mention of Exploit Weakness, so there is one point for Fortitude. Otherwise the wording is similar: can't counter powers, and powers only.

The only real difference is that last line about "this team ability can't be countered." That is important for a couple reasons. First, the feat Dissent allows characters to counter TAs as if they were powers, so both Fortitude and Guardians would prevent Dissent from countering TAs (since you can't counter powers of characters with those feats/ATA). On the other hand, some characters have SPs that just straight up allow the counter of TAs (and the phrase "like a power" isn't mentioned). For example, take SI Yellowjacket's TRAITOR:

TRAITOR: Yellowjacket can use Outwit. He can use it normally, or he can choose a team ability possessed by an opposing character 6 or fewer squares away to which he can draw a clear line of fire and counter that team ability on all opposing characters.

See? The word power is never used. Fortitude would not prevent TRAITOR from countering TAs... but because of that last line in the GoG ATA, TRAITOR could not counter that TA.