Thanks Billy. I've just started learning more about the people here so I can't comment on IceQueen's statement but I think we all can go back and forth between acting silly and being mature. Just depends on the situation.
The sad thing is I feel like I had SOOO many lost opportunities with the nice guy thing. I've had times in my past college years when I KNEW that someone was checking me out but didn't quite know what to make of it. If I actually came up with the balls to say something, we'd end up talking for awhile and have a good coversation and before you know it, we've talked for an hour or so. Then I find out since she's opened so much into me and i've given back with talking and listening that she feels like i'd be someone safe to trust with info/hopes/dreams/problems etc. Therefore I just became "nice guy" material which =friend. The possibility for BF was lost by me taking an genuine interest in this person. It's like women want you to NOT give them the attention they actually crave. Always letting them want to delve a little deeper into you or hoping you'll find a way into their most inner selves just enough to catch a glimpse, never enough to touch.
I've always been a friendly guy, not too outgoing or the life of the party, but approachable and easy to talk to. I tend to smile quite a bit more than I have to when i'm around people I know. (but I hardly ever smile by myself i.e. walking in a store) I had a big issue with being social in my HS and early college days. Kinda seems like the commercials for Social Anxiety Disorder. I was more happy when people wouldn't talk to me because then there wasn't any pressure for me to F' up in some way. There were times I actually got nervous to the point if an embrassing situation arose that i'd break out in a painfull rash on my chest. Ugh!
I eventually got sick of being afraid and have gradually been overcoming it in time. Went to my first dance club last year and had a good time with some friends. My friend laughed his ass off because (supposedly) some hot girl came up behind me and tried to get my attention to dance with me. I didn't notice her from the way I was facing. Other guys tried to cut in between her and me and she kept manuvering to avoid them and be next to me. (my friend is pretty good at seeing things in the background that I miss) She eventually gave up and went off in the crowd somewhere. After he told me I was kinda bummed out but felt a boost of confidence that I must be doing something right.
The most unfortunate thing is that we learn from our mistakes. I'm still learning how to read people better and give dating other women a chance. I seem to be able to meet people when I actually TRY. I haven't been able to just go up to a stranger and strike up a convo with the intention to date. But I have made myself just talk more often to women regardless of my attraction to them.
Just try it! It starts to work if you keep that nice guy attitude but act like it's no big thing what they think of you. I've had the girl at the food counter of the local bowling alley remember my name after one order and a 2 week gap and felt some good vibes coming from her just from general chit chat like "how are you doing tonight?" I had no intention of pursing or anything since I have a gf but the social interaction with the opposite sex is a good thing and it wasn't disrespectful seductive or anything. Same thing happened at a salon.
This time though I specifically decided to talk and be aware of responses and reactions for an educational view on the "nice guy" approach. Got my hair cut and made small talk with this gorgeous stylist. Asked her about work, her schooling, if she thought my hair style looked good on me, cracked a few jokes and made her smile. She have me her card and said how nice and fun it was to meet me and that I ask for her when I come back. Now sure they might tell other clients that but I definatly felt some flirtatious vibes from her.
For some reason I felt that she was a higher caliber than i'm used to so when we talked I acted like a nice guy, but I was a little sarcastic and threw some minor bragging kinda like a little bit of cockiness tossed in there. It seemed to get her attention in a good way.
So all i'm saying is just be yourself. I look at in in the way of a person's body. What their body is, is what they really are. My body is a "nice guy" body. BUT you can change how you dress. I can throw on a shirt of sarcasm, and maybe slip on some "Nike:I don't give a damn about what others think" to appear a certain way, but what holds it up is the "nice guy" body. I think it just all depends on what you want people to FEEL from you as opposed to just SEE in you.
If you truly are a nice guy. You can't just remove it. You can hide it but the truth comes out sooner or later. If you WEAR different attitudes and emotions, women will believe you're capable of all these types of behaviors and feelings and therefore find you exciting in that sense. You won't really have to be the "bad boy" to keep her interested.
(I'm finding this out the hard way in my latest relationship. She seems to be getting less passionate since i've put alot of "care and concern" into her. She actually pushes for attitude from me because we don't argue or fight. She initially told me how she's been hurt and etc, so i've treated her real good but I forgot how women like all their emotions touched, even the bad ones less the don't feel completely alive. But every woman is different and some feel the need less or more.)
(but you should never deliberatly be evil or hurtful to someone just to tap an emotion btw)
Women want to feel that by being with you, they will grow and discover more of themselves on an emotional level. I've made the mistake of always just making them happy happy. But they actually like to feel sad at times, or angry/jealous/bitter/confused. It's one of the things that makes women exciting and unique compared to guys.

(But frustrating too of course! lol)
Sorry about the length but I just wanted to throw out my learnings and experiences.