Satisfying storyline, huge amount of content, plenty of replay value, lovely graphics, and addictive multiplayer.
The game takes place 25 years after the original Kid Icarus, when you defeated Medusa and save Skyworld. Zoom forward 25 years, and Medusa has risen once again, vowing to destroy the planet, the gods, and all that other evil stuff. Naturally, you should stop her, and restore peace to the world.
You play as Pit, and there are 3 main parts of battle. The first 5 minutes of action is in the sky, and with the Power of Flight, you fly to wherever it is your going, and all the while shooting and dodging hordes of Underworld monsters, many recognizable from the past games. The air battles, after getting the hang of the controls, has a strong resemblance to Star Fox.
Then you get on the ground. Ground battles, like air battles, it involves taking down loads of Underworld enemies, and dodging shots. Unlike air battles, the ground stage is free-roaming, has hidden passages, and you may even find a puzzle or two in order move on.
At the end of the ground battle there is always a boss. You play just like any other battle, except, well, there's this one huge freakin' bad guy that's trying to slaughter you.
This all looks like any in-the-norm game right now. But what get's you really into it? 1: There seems to be constant banter going on, whether it's Pit, Palutena, a Boss, or some other living being. The chatter is filled witty comments, bad puns, and just plain funny chit-chat that's sure to crack a smile (or scowl in disgust, if your not into that sort of thing.).
The other main reason, and probably the biggest one as to why I keep playing, is the weapons. There's huge variety of weapons. There are 9 weapon types. Each weapon type as at least 12 different weapons. And all weapons have modifiers, such as more charge shot damage, or possibly a status effect. Also note that each weapon has a stat in range and melee damage. You can collect weapons in solo mode replaying stages and adjusting difficulty for stronger weapons. You can also get weapons in multiplayer, whether online or local.
Speaking of multiplayer, that's the next thing. There are 2 multiplayer modes. Light vs Dark, which is a 3-on-3 team battle. The other is free-for-all, which you can probably figure out by its name. There is no actual ranking system, but the game does record the amount of games played and amount won.
This covers the majority of the game, though others features that shouldn't be left out is getting weapons through StreetPass and SpotPass, AR cards that can battle each other, and idols, which are much like trophies in Smash Bros.