I guess this is just semantics, but in my book 'aggressive' is by definition a bad thing. For the traits you cite I'd go with 'assertive' and/or 'self-assured'.
Aggression, in its broadest sense, is behavior, or a disposition towards behavior, that is forceful, hostile or attacking. It may occur either in retaliation or without provocation. In narrower definitions that are commonly used in psychology and other social and behavioral sciences, aggression involves an intention to cause harm, even if only as a means to an end. It has alternatively been defined as acts intended to increase relative social dominance.
Not exactly an endearing definition. Dictionary.com has even less flattering definitions.
Passive (or passiveness) isn't the the antonym of aggression. Here's a few that I found: agreeableness, friendliness, friendship, gentleness, kindness, niceness. I would much rather be attributed those qualities.
It really depends. You can be forceful in getting your work done. You can be hostile towards procrastination. You can be attacking big problems. Ultimately, its just a characterization of behavior. I think context and where that attitude are directed have more bearing on whether its 'positive' or 'negative.'
The opposite of aggressive is passive. Both are bad. What you want to be is assertive, which is the middle road where you look out for your own interests while still respecting others.
There're a bunch of other situations where both extremes are bad. The opposite of indifferent is obsessed, and both are negative qualities in a relationship - you want to be involved. The opposite of flat is sharp, and both are bad - you want to be in tune. The opposite of hypo-anything is hyper-anything, and both are bad - you want to be healthy.
