At first I thought the whole change was a gimmick. But after reading some of the Thor comics it got me thinking
 

Do Asgardians recognize people's souls not their body
Loki comes back as a female, inhabiting the body of Sif, when people saw him/her nobody really said "hey look its our friend Sif" instead they could see "Loki the trickster"
 Another example would be when some UN peacekeeping troops are in Africa, they are trying to protect poor people from a bunch of thugs with AK's and rockets. Their bodies don't look like Norsemen but Thor recognizes their behavior and bravery.
 
 
He flips back and forth changing sex but people see the same god of evil and trickery?

Actually I think it's just because they're used to Loki being the shapeshifter and all together god of crazy. lol.  But really it's a valid point, but I think they just got used to it pretty quick because Loki is notorious for being the trickster. As far as recognizing the others I think that's all Thor, because of how long he's lived and how well he knows the other Asgardians.

Well wearing the exact same clothing, complete with giant horned headgear might have tipped them off...
 
Thor not so much recognized the warriors three, as fit in the details of who they are. First time he met Heimdall he didn't recognize him at all.

Being an ass doesnt stop by just being a different gender. His douchebagness was very obvious

I think it was his lack of eyebrows. Everyone knows only Loki would be evil enough to shave off Sif's eyebrows. 

Most people forgot that Loki was ranked as the most powerful mage in the MU.  Loki was such a powerhouse he toyed with Doctor Strange.  He went to realms univited and was not humbled.  Spells to cloud recognition are simple for Loki. He caused Ragnarock, he needed a ploy to blend in, gain some trust and do what he do.  The  god of mischief.

@AtPhantom said:

Well wearing the exact same clothing, complete with giant horned headgear might have tipped them off... Thor not so much recognized the warriors three, as fit in the details of who they are. First time he met Heimdall he didn't recognize him at all.