Metal Slug sacrifices some depth to focus on being a visual thrill ride with jaw-dropping 2D graphics and animation (by 1996 standards.) It's thus actually a bit of a shallow game experience, but odds are you really won't mind. That's because the whole concept is pulled off with such chutzpah and skill, it makes you feel quite a bit more lenient than usual toward the game's flaws (some of which are actually kind of serious.)
The premise, first - two goofy soldiers, something of a caricature of Contra's ultra-macho Bill and Lance, are charging through some generic evil army of enemy grunts and just totally blowing hell out of them. Presumably there's some world-saving going on in the background or something that serves as justification for all this carnage, but SNK doesn't see fit to elaborate on it in-game. The grunts plow through a multitude of levels facing seemingly never-ending waves of enemy soldiers, starting out with a rather weak pistol and salvaging weapons upgrades like the Heavy Machine Gun and the dreaded Rocket Lounger along the way. The "Super Vehicle" in the subtitle also references little mini-tanks that you can pick up, which absorb several hits for you and increase your firepower, and also grant you a 10,000-point bonus if you have one intact at the end of a level. There's also various Hobo Soldiers imprisoned along the way that you can free, who cheerfully offer up bread and dead fish that they've been stashing in their loincloths for probably a pretty long time.
The main draw of the game is the lavish animation. The enemy soldiers are usually unaware of your presence as you approach, and are going about their daily routine of washing their briefs or roasting a Hobo Soldier over the fire while telling stories or whatever. Engage them in combat and they have a surprising range of moves - they'll fire back, lob grenades, try to charge you with a bayonet, or possibly just run away in terror. They also have equally elaborate death animations for all the different weapon types you can hit them with, from being painfully roasted alive by the flamethrower to being rendered into gory chunks by a rocket. The backgrounds feature some very nice hand-drawn art and always have some little things going on, from minor characters conducting shady deals to a flock of parakeets flying through a jungle. The game really features top-flight art and 2D sprite work for the time, made all the more welcome by the fact that it came out when ugly blocky 3D polygons were all the rage and much of "games journalism" was crowing that 2D was outmoded and about to die out for good.
The gameplay is solid, if not spectacular, but likely you'll be so entranced by the visuals and music (a funky combination of military marches and jazz fusion) that you'll remember it being much better than it actually was. My main point of contention with it is that the level design is really uninspired, covered up by the overlay of the beautiful graphics. If you can see past all the shenanigans going on with the sprites, you'll notice that the levels are really usually just a flat plane with maybe a few ledges to jump up or down on here and there. Another bone of contention is the inability to fire diagonally with most weapons (and even with the weapons that are capable, like the Heavy Machine Gun, you can't fix fire in a direction a la Contra but rather have to sort of awkwardly flip up and down to sort of spray in the general direction of what you want to hit.) The technique of creating difficulty in a platformer by limiting the heroes' field of fire and then making enemies come at them from weird angles that are hard to adjust to is a very old, old, dusty and musty technique, dating back to the first Castlevania game and beyond, and it should have died a death due to obsolescence long before Metal Slug came out. And yet, here it is again, this sad quickie substitute for designing clever and challenging level layouts.
There's other little nitpicks - the Super Vehicles have an equally bizarre means of firing that forces you to sort of retreat and move awkwardly to get them firing forward, and the Hobo Soldiers that you save are not counted toward your final tally at the end of each level if you merely lose one life (which seems a bit of a slap in the face for an arcade game designed to kill you off as much as possible.) The way the controls work, it almost seems like this game was originally planned for a two-joystick layout something like Ikari Warriors or Smash TV, but then someone reminded the design team (after the game was like 90% completed) that Neo-Geo cabinets didn't work that way.
The game just ends up being so cute and lovable, however, that I can't see most people minding any of that all that much. This was a great one to flip a few quarters into in the arcade back in the day, or fire up with a friend now in MAME, but if I'd paid top dollar for the home version of it back in the day I get the sense I'd have felt a bit ripped off.
* Gameplay Video