Choosing a handlebar width Are there any rules/tips to choosing a handlebar for your bike?

I live in a big city, and I've seen fixies with really, really narrow handlebars (presumably to avoid hitting pedestrians?)

I personally like the way wide handlebars feel (especially when riding fast), and I'm wondering what, if any disadvantages there might be to having a ~28" bar on a non-sus mtb for city riding.
Let's unpack that a bit. 

The first consideration is control. 

Do wider or narrower bars give better control? Narrow bars mean that small movements cause larger direction changes, wider bars mean you have to move them more to steer. The amount you need to turn depends on your speed. If you're doing 30 kph (~20 mph) then you need only small steering adjustments, but if you're weaving through stationary traffic you could be making much larger turns. 

On a road bike where you'd often be riding faster, it turns out that the most comfortable width is around the riders shoulder width. But you'll notice that roadies tend to have less fine control at low speeds. On MTB's, people often want wider bars to get more control in heavy conditions. Wider bars also give better control at low speed. So the minimum width for a MTB has the inner edge of the grips at about armpit width. As a matter of preference you can go wider, but as you do you should shorten your headstem to compensate.

The next consideration is traffic. 

Are you the kind of rider (like me) who rides through gaps between things (cars, cyclists, poles)? If so, then narrower is better. (I suggest to never ride close to our between pedestrians, they are too unpredictable). 

So, the downsides of wide bars


they upset your bike fit, so you should use a shorter head stem. 
they limit the size of gaps you can ride through.