The pants you wear depends on your body type. If you are thin, then you definitely want slim cut pants. But, there is a difference between slim and skinny. Skinny is more hipster-ish and even "gay." But, for a skinny guy, slim pants just look like pants that fit well.
You want clothes that somewhat hug your body and show off the shape of your body. If your clothes fit too lose, it makes you look more skinny if you are skinny, or more fat if you are fat. Too many guys try to wear bigger clothes thinking they can hide their body's imperfections in them. Doesn't work that way, it only makes you look worse. Too much fabric is bad.
Last year, Men's Health had a special edition to "Build Your Perfect Life!" Jeans was one of the things they talked about and which cut to wear for specific body types.
Here is the simplified list, based on body type:
Athletic - Straight leg, Athletic cut, Boot cut,
Average - Straight leg, Boot Cut
Stocky - Athletic Cut, Relaxed fit
Thin - Slim straight, Slim boot
I used to not pay any attention to the cut of my pants, only the size. I was wearing athletic cut, relaxed fit, boot cut... anything. And I was comfortable wearing them and thought I looked good in them. I thought I was hiding my skinny legs. Then, after researching a bit about proper men's fashion and learning that the fit of your clothes is the most important thing, I decided to give a pair of slim fit jeans a try.
At first I felt really uncomfortable in them. I thought they were too tight and everyone would see how skinny my legs were. But, after wearing them for a day or two, I began to feel really comfortable in them. I began to like how they looked and realized that they didn't make my legs look skinny. They actually gave my legs some shape and fit my body really well.
Then when I tried to wear one of my old pairs of jeans, I realized how ridiculous I looked in them and poorly they fit me. I had no shape in them and they made my legs look like flag poles. Straight, skinny, long, and with baggy pants as a flag. I gave away all my old pants to goodwill, and bought a bunch of slim cut pants.
I've found that Levi's 514 (slim straight) and 597 (slim boot) are the perfect pant for my body.