One of the things I love hearing from my teams (usually through a third party) is that they think I am a "nice" manager or "nice" person. One of the things I hate hearing (usually from my peers or my bosses) is that I'm not as effective a manager as I could be because I am "too nice". To me, this all comes down to how you treat the people who work in your teams and how they respond to you.
We've all worked for "not nice" managers and bosses. These are the people who think that intimidation, grumpiness, pushiness, and generally obnoxious attitudes are the way to convince your team that you mean business, and that if they don't respond the way you want them to you'll be even meaner! My experience as an employee working for those types of managers is that they never motivated me except to find a way out of their group! It was always the managers who encouraged, communicated, lead by example, and became part of the team that motivated me and caused me to put in the extra effort. And I wanted to stay in their teams.
As I rose through the management ranks, I tried the mean approach a few times, tried the authoritarian approach once or twice, and tried the pushy attitude a half dozen times and realized the people working for me had the same attitude I had to the nasty managers: they were less motivated and abandoned the team! In only a few short experiences I realized that this image of the a-hole boss may be something we see on TV or perhaps in the general labour pool, but when dealing with educated professionals, it is a complete waste of time and never leads to positive results.
That doesn't mean you have to be fake and be "nice" when you're not a nice person. Just that you have to let your own personality shine through. Few of us are a-holes by nature. Why change just because you have a position of responsibility? Having managed teams and companies for decades I find I am the most effective when I am simply myself. By nature I'm pretty mild mannered, easy-going, and friendly. Just because I become "a boss" doesn't mean I have to change, does it?
Sure, there are times when you have to be tough, or demanding, or pushy. But those attitudes are warranted by the situation, not in general. On the whole teams respond much better when you're part of the team, not conflicting with it. And when you look at the most effective companies, the most productive teams, and the most loyal employees, they work for managers who are part of the team, supporting and encouraging, participating and promoting. Instead of belittling, insulting, and screaming. No one will work for long in that environment unless they have no choice.
If there's one secret to my success as a manager it's been to create an environment for my teams that I would want to work in. That covers everything from the equipment to the environment, of course, but a huge part of the equation is the way I treat my team members and the way feedback is handled in both directions. There is never a reason to not be polite, friendly, or kind, unless the situation is so extreme a burst of anger is necessary to correct or illustrate the issue. Like many managers, I can fake being mad, upset, or disappointed (and sometimes those emotions are not fake, of course!). But the trick is to use those as a tool, rarely, and only when the circumstances warrant. You'll get taken a lot more seriously when you do get upset if you do it only once or twice a year, instead of every day!
And the people in my teams are professionals. They are engineers or scientists, or on their way to being one of those two, or in a support role such as accounting, legal, HR, and so on. None are Grade 8 dropouts, and all deserve three simple things from me: respect, reasonable treatment, and the chance to always keep their dignity. Give your team those things and you will get it back. Yell at them, often, and you'll get virtually nothing back.