"Party (XXO)" isn't ground-breaking, it isn't making me worship the songwriter or anything, but if you will recall, neither was "Hug". What those two songs have in common though, is that they are the formula, the quintessential idol group song -- done right, and with just that much more musicality. Of course debut singles have to be good, but over and above that, they have to show off the group as well as possible, and open as many doors for the future.
From the very beginning, the harmonies are there to go and say "hey, listen to us, we actually have voices" -- they're pretty generic and cheap around the edges, but the thing to pay attention to is that they don't sound like computer-generated harmonies, nor do they sound like one girl singing all parts just thrown together during post-prod. The harmonies have body, body only a group dynamic can give.
The chorus is gorgeous, and again, it sounds very simple and cutesy, but listen to it closer, those "dips"/"scoops" require trained, talented voices to pull off, and control. Give that to another useless idol group member and those dips won't even exist. The hook is melodic but catchy in it's own way, and again, those harmonies make it so, so much better.
And while all of that is happening, you have one of the most epicly Swedish instrumentals going on -- with bass you can actually feel, and gorgeous yet subtle synths. And really, as a whole the instrumental is extremely subtle, stepping in only when needed, and just sitting back and letting the girls do their thing, and do it well.
Add everything together and listening to it is such a pleasure because from the song alone you know these girls know exactly what to do, and you know they love it. There is nothing more effortless than talent laced with conviction.
This, I believe, is a slap in the face to every female idol group who ever tried to do soft and feminine but confident, and failed. GLAM could very well be my new favorite girl group, if they manage to keep this up.