There must be really specific reasons why it sucks. Some have hit on that the controls are a little loose, which for me means your guy moves in slow motion while enemies charge at you from out of nowhere.
The graphics and sound are solid, as well as the premise (rise from your grave and rescue some maiden by morphing into creatures).
The kick the white wolf for powerup idea doesn't work for me. From what I remember, if you miss one, which is pretty easy if you are me, you have to repeat the whole board over again, which is lame.
I think the destructible platforms rising and falling on the first level, which sounds cool in theory, adds to the frustration and futility.
The bosses aren't fun to battle. Why? Is it because they are cheap? Are they too big? (I'm sure one of the selling points of the hardware were the ultra large sprites)
There are lots of games with flaws that people play over and over again. Why does this one just leave a bitter taste? Is it specifically because it was the Genesis pack-in game and so many were forced to play it over and over again?
Also, what's with the vaginas that you kick on level 2?