Yes, I agree. The "nice guy" is the one who's nice and totally non-threatening, i.e., non-sexual. Being self-assured - and humble (I find a degree of self-deprecation helps), but also clearly being interested is key to not being dismissed.
I have to disagree with this part of your post. Being a nice guy does not mean that you are non-sexual and not interested. Being self-assured does not mean you are not nice guy.
Now, I *am* a pretty nice guy. When I was dating I had plenty of luck. I guarantee you that I was plenty self-assured and definitely showed interest. These things are not mutually exclusive. When you (the general you, not you specifically) start thinking they are mutually exclusive, that's when you start getting assholes who think they're looking confident!
Mr Awe