These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: Metal Slug
Awesome Art: The amount of detail put into the sprites is very impressive.
Good Bad Bugs: The AES version of Metal Slug 5 had an un-dummied, incredibly easy-to-access debug menu which allowed to play any part of any stage on demand, plus a bunch of test areas full of free weapons, vehicles, unused stuff and a sprite/animation viewer.
Allen O'Neal is no slouch in this department either.
I'll see you in hell!
Moe: The amount of blood spilled, flesh torn and bones crushed is usually proportional to the cuteness of your mech.
There's also Fio, the series' resident Moe character.
Most Wonderful Sound: "You're great!" Gained from racking up a ton of points at the end of a level from rescuing a ton of PO Ws without dying and possibly clearing the mission with a Slug still intact.
"Mission Complete!" followed by the Mission Complete theme.
A lot of the deaths are animated in disturbing detail. You may want to avoid lighting people on fire.
Replacement Scrappy: Trevor and Nadia, as they replaced series' regulars Tarma and Eri in Metal Slug 4. The latter two actually appear in order to justify stage transitions (they're driving the vehicles that pick you up at the end of every level), but that's just not enough.
Scrappy Mechanic: Metal Slug 5 introduced a Mega Man-like slide move. Though it was useful at times, the problem with it was that it was activated by holding down and pressing jump. So most of the time when you wanted to jump and shoot downwards (a common tactic to shoot things underneath you), you would slide instead, and probably die, or at least waste time. Thankfully, it was taken out of future installments. But once Damn You, Muscle Memory is overcome, the slide is an absolute blessing.
So Bad, It's Good: The game is very aware that it emulates trashy action movies from the '80s and the '90s, and the effort put into the storyline is either non-existent, or results in heavy dose of narm.
The bosses of stages 3 and 4 of Metal Slug 6 are much more durable and nasty than the first two bosses were. Both take ridiculous amounts of punishment and have nigh-unavoidable attacks. Expect to use a lot of credits.
The stage 3 boss deserves special SPECIAL mention. Holy fark, that robot... First: You're moving to your right, constantly, which means it feels awkward reaching around and shooting at the robot, not to mention the controls automatically face you to the right if you try to jump to the right to dodge something. This means it will take forever to kill it. Secondly: it keeps attacking, almost non-stop. And all of them are constant, even with the obvious pattern, so you'll be killed repeatedly. The "vulcan fix" skill is very useful here (hold down the shooting button to keep shooting in a fixed direction). Also, don't even try to take your time in defeating it, or it will get pissed and keep swinging his electrified arms reducing the safe zone to a ridiculous minimum!
Suspiciously Similar Song: The mission 3 music of Metal Slug 2 has a sax solo taken from the shooter Air Duel. The underwater theme of Metal Slug 3 is a remix of the stage 3 music of Air Duel.
The Desert theme from Metal Slug 3 bears more than an uncanny resemblance to Iron Maiden's Powerslave.
The final mission of Metal Slug 3, which is the longest mission in the entire game.
To make things even worse, if you're playing on the Xbox port, you have limited continues, and if you use one, you'll start back all the way from the beginning.
They Just Didn't Care: On the official soundtrack for Metal Slug 4, several of the songs are given the wrong names and played out of order. In earlier copies of the actual game, the final boss's theme was switched with his intro theme.
Ugly Cute: The Mars People. They're adorable, in a weird sorta way.
Unfortunate Implications: You start Metal Slug 2 in one of the most stereotypical Middle Eastern villages ever. Then there's the recurring, machete-wielding islanders.