It really shouldn't be a matter of opinion whether a game is a shmup or not. It either falls into the defined characteristics, or it doesn't. However, just like the many anomalies of the English language, so too do the rules of defining shmups (or any video game genre for that matter) delve into the realm of uncertainty.
First off, a "shoot 'em up" is more of what you do in the game, not what type of game it is. The same goes for "first-person shooter," which is simply a point of view and what you do. Calling Resident Evil a "survival horror game" is halfway there, though the survival part is just a tacked-on noun (and honestly, there are hundreds of other games that require you to survive, but don't mention it in the genre). I'm not even a fan of calling games "role-playing games," as I naturally enjoy playing the role of whatever character I'm playing, even Dr. Mario. I understand these genres put extra emphasis and dedicated TLC into their respective areas, but those same characteristics can be found in many other games. In these examples, it becomes clear that we can't simply use the literal meaning of a genre without getting into fisticuffs with ourselves.
That's why I find it interesting that there are games credited as - without a doubt - shmups, yet I can't help but disagree. And then there's shmups that shouldn't be deemed so, but have me welcoming them into the genre with open arms. Take for example Rez, which most definitely has you shooting things up. In fact, it's the only action you can do in the game other than moving across an X-Y plane. Yet I can't bring myself to call it a shmup. Something doesn't feel right about letting it into the genre, knowing all of the other fantastic characteristics that require mentioning. Calling it a shmup removes all of that effort made to hook up the soundtrack to your heartbeat; it makes us not wonder why there's so many different visual styles in which to play the game in one continuous sitting; it makes us not question the purpose of the hard-earned Trance Mission mode. To put Rez in a genre already occupied by so many dissimilar games would be inappropriate. I wouldn't dare place it somewhere it doesn't belong and I won't even mention the "s" word (but I'll gladly link to it). Also of note is how Space Harrier is near-identical in what you do, but I have no problem calling it a shmup.
On the other end of the spectrum, we have Geometry Wars 2. How can this game not be a shmup? You play the role of an abstract ship that flies around, can shoot out what look like bullets, can detonate bombs that clear the screen, and have various enemies to shoot until you run out of lives. That passes my test. Now let's take a look at one of the six gameplay modes, Pacifism, in which you have no bombs, cannot shoot, and must simply dodge the never-ending swarm of quadrilateral foes while collecting multipliers for a high score. This cannot be a shmup, for you physically can't shoot anything, and yet it still feels like a shmup. Granted, it helps that you can shoot things up in the other five modes, but this example brings me finally to a look at Squares 2, by Gavin Shapiro, in which you play the role of an abstract shape that cannot shoot, has no bombs, and must simply dodge the never-ending swarm of quadrilateral foes while collecting black squares (and sometimes circles) for a high score. Sound familiar?
There have been games like this before, including the popular GBA homebrew game bulletgba, which is credited by the author as a "Bullet-Hell Simulator." However, those are bullet patterns right out of famous shmups, so it's too easy to associate the two. I'm focusing on Squares 2 because of how distant it is from the genre, yet it feels right at home. The game itself is not just my favorite addicting Flash game on the Internet (a simple search for "squares" in Google yields the game as the first result, showing a global popularity).
I find shmup charm in Squares 2 in how you have very limited time to react to on-the-fly patterns of things to dodge, all the while trying to collect as many black squares and circles as possible (thus giving you a reason to move and a motivation to survive, other than that shiny high score). And who cares if you have mouse-precision control over the shmup-traditional arrow keys / d-pad / joystick? Who cares if there's no end to the game? It's an endurance test (unless of course you're a cheater). The design of the game also helps out in its success: eye-catching red objects must be dodged, circular powerups / powerdowns stick out like sore thumbs, an annoying and undesirable buzzer sounds upon your failure, and the addictive audio crack that is Daft Punk plays on loop forever as you struggle to outdo yourself.
It's a beautiful formula that works to improve both your reaction time and how quickly you can interpret rapidly-changing patterns for the purpose of survival (a skill most definitely transferable to a traditional shmup and even reality). Keep in mind that the game is not perfect. The two biggest factors that take away from its appeal are how red squares sometimes spawn under black ones (quick fix: always have red squares at a higher depth than black ones) and how there are red circle powerdowns at all. If I had the choice to either touch a red square (knowing it ends the game immediately) or touch a red circle (knowing I'll continue with the game but suffer a minor penalty), I'll always choose the red circle. Why, then, are the red circles treated as these special-case "watch out for us!" entities, when it's the everyday red squares that are the real threats? I suppose only Gavin himself can fill us in.
The point I'm trying to make is that putting a game into one genre can be tricky and oftentimes unjust (a point also made by Mike Lopez from Gamasutra in relation to the racing genre). I like my shmups top-down, vertically scrolling, with lots of explosions, and a memorable experience that makes me wish I had more time in my life to perfect a playthrough... but that's just me.
Normally, I'd link to a video of the game at hand for those unaware of how it looks in motion. This time around, it's much more appropriate to just post a link to the game and have you play it yourself.
Speakers up, mouse area clear, and eyes alert: Here's Squares 2.
[cross-posted to Gamasutra]