For me, the addiction comes from how it distances itself from other shooters. You are so much more mobile, and can be stealthy without camping. Also inking turf gives something to do other than wander around the map looking for kills.
For me it's because I pretty much always get to do the most important part of a shooter, using your gun (even if it's not to shoot someone). Besides, any game that lets you use a minigun equivalent as a main weapon deserves my attention
I'll just agree with both of these. I get so easily bored in other shooters. Half the time you're just wandering around, and when combat DOES happen against an opponent that you've encountered, it's over in 2 seconds. Sometimes with BOOM HEADSHOT!!!!!!111 which I've always found to be an annoying mechanic. Mobility is usually low, there's rarely any emphasis on it, and 95% of the time, you're not actually shooting anything.
I grew up with games like Doom and such, which emphasize extreme mobility, speed, and actually dodging things, instead of doing what I call "hide & snipe", which seems to define basically every FPS out there these days, except for this game and Team Fortress 2 (which is absolute chaos). Yeah, you CAN camp a spot in this with an E-liter and just pop opponents if you so choose, but you dont HAVE to play like that with every damn gun, and opponents can actually find you via the aiming beam instead of being constantly popped by an opponent that they never see. It also means they can dodge, which is actually INTERESTING.
And of course those older games also emphasized CONSTANT use of all of the game's mechanics, which definitely involved shooting stuff. Blasting things is half the fun of it. Splatoon encourages me to shoot literally everything, which is the most fun I've had in one of these in a long time. My natural inclination in most games (particularly fighters) is to charge at stuff and go berserk (or in the case of the Splatling, stand there, hold an area with constant firing, and laugh maniacally), and unlike most shooters, this one lets me actually do that.
Not to mention creative and interesting weapons (a freaking bucket! A paintbrush!) and subs/specials. I dont like the Inkstrike one bit, but I do like pretty much everything else.
I really hope to see MORE creative and interesting shooters like this. This is the first one to give me that Doom-like rush of chaos in awhile. Well, there's been other interesting shooters, but they're usually focused entirely on single-player and often are PC-only.