Though I haven't played it to the end yet, I highly recommend the 1991 Sega Genesis title Rolling Thunder 2 to all action-game fans. It just became available as an $8.00 download via Wii's Virtual Console. Someone on a Virtual Console message board said that this game is like a 2D Gears of War in the way it forces you to balance attacking with diving for cover. I thought that was great.
Not a straight-up port of the arcade game, it's an effective home-console adaptation, tweaked to encourage longer play sessions. There are lots of hidden power-ups that extend your dude's life bar past its usual amount. Its level design is pleasantly varied; while most of it is pure twitchy action, it pauses now and again to get surprisingly puzzley.
While your character can leap around in a typical video-gamey way, he's quite fragile (usually dying if shot once) and can only fire his gun while standing still. This lets the game present you with situations where the particular terrain forces you to stop and think about the best way to approach an enemy before he gets a chance to blast you, since charging them head-on won't work. (Note this when you're fighting those ridiculous blue bastards who roll bombs Donkey Kong-style down the stairs at you.) For a side-scrolling jump-n-shoot game, this is very good level design.
It's also got a two-player mode, and I have no idea how well that works yet. The single-player game alone is worth the price of admission.
Once again you have downloaded a Genesis game that I own in its original cartridge form and have fond memories of playing way back in the day. I believe I paid the full retail price of $60 when it was new. BTW, are you sure it's not a straight-up port of the arcade version? What are the differences that you know of? (Not that I would know; I think I've only seen RT2 in an actual arcade once in my life. RT1 was much more common.)
I'm going on memory, but I once played RT2 on MAME and it was a completely kooky game, the original RT with all this extra stuff thrown in. The Genesis RT2 seems a lot closer to the original arcade game, just with a two-player mode and different levels.