If you go in to Metal Gear Solid expecting top gameplay, especially the older ones (you know, that first one they released, you know, about 13 years ago?) you are foolish. I, personally, think the gameplay is fun, and I especially love MGS3 and 4 for their stealthier aspects.
Coming from someone who has completed the entirety of the series, sans Peace Walker, which I still haven't gotten my hands on, and 3, and 4, which I completed without killing the single person, I love the gameplay. But it's not something to write home about.
You play MGS for the style. For the cinematic-like presentation. For the convoluted, crazed, ridiculous, and campy storyline. For the deep, profound, if preachy, social commentary. For the utter badassery that seeps from Solid Snake, his father, Big Boss, and other characters. It's a full package. From the way you put it, you played MGS4 to shoot up people like most modern AAA titles. You didn't like it. Fine, more power to you. I loved it, and as of 7PM today, completed MGS1 for the 5th time. More power to me.
EDIT:
I must say, I am rather disappointed that the poster above me calls himself a MGS fan. Also, I'd like to know where he was able to obtain a copy of Rising, as he comments on how crappy a game it is. I'd love to play that early.