I'm not fussy about most things on a bike, but I am fussy as hell about handlebars. So here's my rant:
The straighter the better, as far as I'm concerned. Too much sweep makes my shoulders feel cramped and awkward.
Too narrow does the same. Anything under 800mm is uncomfortable for me. 780 is virtually unrideable.
Not much of a fan of high bars, especially on MX bikes. Being reasonably tall, high bars stop me moving forward on the bike.
Lots of older bikes need higher bend bars - the Renthal 764 bend is the best I've found. The Pastrana bend is high, but has way too much sweep.
And beware rotating the bars forward to try to make more room - if the bar ends are noticeably pointing upward, then you'll ride like a goon. It's difficult on most older bikes, but moving the bar mounts forward is MUCH better.
I have wondered about making up some bars for old bikes that can be angled forward to give a more forward riding position, but leave the bar ends level with the ground.
Aluminium bars take the "sting" out of hard landings and can take the edge off vibration. Fatbars do both of those things even better, but I think they look crap on old bikes. Steel bars are the work of satan...
Renthal and ProTaper are the strongest in my experience.
Tomaselli and Tag are nearly as good.
Reikons are as soft as butter - not noticeably better than the cheap garbage bars.