^Ah, the ways Streets of Rage 2 bends the competition over a barrel without sending it flowers afterward...
1) Obviously I'd be remiss to not mention the amazing soundtrack.
2) The gradual difficulty curve. No other beat 'em up to my knowledge offers this. Most of the time, by the end of stage 2(a lot of the time by the end of stage 1) you'll have met every non-boss enemy in the game. Here, you're still meeting new, tougher enemies on stage 5,
then the game mixes it up for the final 3 stages.
3) Effective weapon pickups. From pipes, to swords, to knives, they're all damaging. More on this a bit later.
4) The animation improvement. The original game had your characters eventually do something every so often if they were idle. Here your characters are entirely animate when idle. The first I can think of in a beat 'em up, since Street Fighter 2 had come out.
5) The move set. This is what makes or breaks beat 'em ups, for the most part. Here you have:
*A strike combo(B,B,B)
*High chance of guard crushing strike combo finisher(hold B for a couple seconds, then let go)
*Backward strike/throw weapon(B & C/hold B as you press C)
*Jump attack(C, B)
*Forward jump attack(forward & C, B)
*Downward jump attack(C/forward & C, down & B)
*Blitz(forward, forward & B)
*Front grab(walk into enemy) strike knockdown(B)
*Front grab strike combo(hold forward, B, B, B)
*Front grab throw(away & B)
*Front grab vault/Jump(C)
*Back grab throw(front grab vault/grab from behind, B)
*Desperation attack(A)
*Alternate desperation attack(direction & A)
*Land on your feet when thrown(hold Up & C until you've landed on your feet)
That's not counting moves that are exclusive to characters. More on that in a bit.
6)The playable character variety. Each character almost feels like you're playing a completely different game.
Max has brute strength the likes of which I'm not sure Mike Haggar even reaches, but Mike most likely moves faster. However, Max can make up for that with his blitz, which moves him a good distance across the screen, and is a decent attack, but not as dependable as other characters blitzes. His front grab strike knockdown is actually a series of punches(which looks like he's snapping their necks) that depletes health bars faster than any other move(though it can be interrupted). He also can only grab enemies from behind by walking into them, but he has a front and back jumping throw. Also, with longer weapons, Max can hit enemies that him from behind just by attacking, whether an enemy is in front of him or not. Honestly, in a way, he feels like you're playing as a boss character of your own.
Axel is a little less damaging than max, but also faster, and can really depend on his blitz, the Grand Upper, to get through most situations.
Blaze is less damaging, but faster than Axel, her backward strike is a foot sweep that hits enemies surrounding her, and though effective, her blitz is harder to pull off. She can also be more effective with a knife than a longer weapon, as she swings a knife twice. Timed right, that can do more damage in her hands than a pipe or katana.
Skate is the fastest, the only one that can run, his blitz is incredibly powerful, if you can aim it just right, but it's also one of the easiest moves for enemies to counter. His back grab throw is actually a series of punches that is almost as powerful as Max's front grab strike knockdown. He has a mid-vault throw, which is possibly the most controllable throw in the game. He also noticeable struggles with larger weapons.
7) The number of enemies on screen. Arcade beat 'em ups often toss 7+ enemies at the player, which usually isn't something a strike combo, or well timed throw can't handle. For a 16-bit home console beat 'em up, you often fight 6 enemies at a time, especially with 2 players, putting 8 characters on screen at a time, on top of up to 3 breakable objects, with no sign of slowdown.
8) Secrets. Well really, just a few 1-ups hidden by the background/foreground(mostly the foreground), but they help you survive once you know where they are.
You put it all together, and most arcade exclusives don't even stack up to it.
Oh yeah, I know some would consider the ability to attack other players detrimental, but it's through doing just that, that one player can cannonball the other into a group of enemies, as they land on their feet, leading to a fun double team. If simply beating the crap out of an enemy from both sides doesn't suffice
.