Harvard's Amy Cuddy makes a compelling case for making a first impression as being friendly and competent.  Of these, friendly is far more important.

Her thesis, explained clearly and coherently in the Nov-Dec 2010 Harvard Magazine, is that our first impressions of people are derived from our fight or flight instincts.  In a nano-second, we determine if the person we're meeting is a friend or a foe and competent or incompetent.

If the new person is friendly and warm, we relax.  Nice if they're competent – we'll admire them.  If they're incompetent, we'll pity them.  Either way, we're not greatly worried.

If the new person is not friendly, but cold, warning bells go off.  If they are a potential foe and competent, we'll at least envy them and perhaps fear them.  If they are a potential foe and not competent, we'll have contempt for them and think about ways to take advantage of them.

Given that, it is clearly in each of our best interests to have others think we're friendly.  Don't know about you, but I'd much rather be admired than either envied, pitied or held in contempt.