The music. Because of it, right off the bat I was hooked. I've always been fond of the chiptune style of music, and that slow volume build at the title screen caught my attention hard. On the gameplay, it does indeed feel like the start of a larger adventure, and more accurately a test to see what you could do with the mechanics. Given that, the necessity to actually fight the monsters was null. The only time I needed to use my gun was to defeat the boss, and I could otherwise run through without throwing a single punch. Also, the need to constantly tap buttons has always been a turn off to me, making the boss fight rather dangerous to keyboards.
But you do have something in store for the larger game, that much I can tell. It may just be a test, but it's a success at that. The graphics tug at my heart due to the era I grew up in, and the controls are crisp like any side-scroller worth its salt should be. Definitely looking forward to what may come next.