Gameplay takes place on an overhead-view almost Bombermanny looking playfield, and all you pilot a little band of dudes around the screen, using A to attack and B to block. As you pummel the crap out of tiny henchmen they join your "line" and start following you around. Gather enough and you can do a "line attack," which is accomplished with a quick flick of the remote. These attacks vary depending on what weapon you have equipped. Initially you do a Contra III style lariat, swinging your buddies around like a giant whip. As more heroes join your party they add more weapons like a spear and hammer which you can swap out on the fly. These also alter your line attacks: the spear makes you hurl your pals out in a straight line and the hammer tosses them in a high arch. To finish the final boss of the demo you have to lariat to trip him up, then switch to hammer and slam the weakpoint from above. Mainly the gameplay consists of whooping the crap out of everything you can see until you have acquired swinging lines of cannon fodder the size of the entire screen, then being pushed along to the next stage.
I mainly heard bystanders say "it's so cute!" while I was playing, which is absolutely true. Everything is rendered in a hyper-cutesy super-deformed art style, and between stages you can listen to the inane garbage that they have to say (I found the idle banter between "Sardine" and "Tuna" to be less than revelatory).
It's simple, it's entertaining, and it's oh-so-adorable: it's Line Attack Heroes, and nobody I asked has any goddamned idea when it's coming out.