It's important to distinguish between being courteous, kind, or well-mannered and being a wishy-washy doormat. People generally associate 'nice' with being the latter.
My partner and I at work (actually, my entire shift at work) conduct ourselves in a very kind, courteous, honest manner, and we're probably the most successful, accomplished crew in the organization. Not to toot my own horn, but it's true. We kick ass on work details and patients love us. My partner is the best example. He is the nicest, happiest, funniest guy to talk to and easy to get along with, unless you try to weasel something or wrong somebody. He doesn't hesitate to let you know.
That's the catch, we don't let people walk on us. Be nice, treat people how you want to be treated, all that good stuff. But when you are treated in a way you -don't- want to be treated, or when something isn't right, you say something. Generally speaking, people respect you enough because of your usual demeanor that they will listen on that basis. If not, well, just stand up for yourself.
I believe the saying goes, "Speak softly, and carry a big stick". This is one of the prime factors that makes an actual alpha male, not just being a meatheaded *******. That bites you in the ass in it's own way.
