I just hit 55 the other day, so I figured I would throw in my two cents. Although I have not done a whole lot of OP's or Ranked PvP, I have some General MMO knowledge from games such as WoW. This is coming from a PvP perspective, because that is mostly what I have played.
My thoughts so far are I really enjoy playing Mercenary, It is just a very fun class personal. Although I believe this class could use some love. I normally play a kiting class in games, for example WoW my hunter was my main. I just enjoy playing keep away and constantly moving. While kiting is very doable in this game, I feel that I can only truly be dominate when I blow my CD's. I.e Power surge. Which should not be the case. I would love to see this class become more mobile and be a true kiting class. I see this class as a support class, it shouldn't' do the most damage, it should bring utility to the group. Which isn't really the case. I don't really feel that any of my utility abilities are anything that another class doesn't have. But down to business. I mostly posting Idea's I have not seen, I agree with a lot that has already been posted.
General-
Make unload usable while moving. This is Arsenals really only snare, we should be able to move with it.
Make Power Shot usable while moving. It is still a cast, we can just cast it while moving like Hunter's Steady Shot. Plus, Arsenal Merc's can get use out of it then.
Energy Shield is so weak, 25% seems like nothing. Reduce the duration to 4 seconds and make it so we have 75% Damage Reduction.
Kolto Overload also seems weak, make it so we instantly gain the 35% health.
A Disengage ability would be fantastic.
At the current state Jet Boost seems broken, classes can resist. Which as of now is one of our few ways to gain distance.
Hydraulic Overrides- Bump up the speed, melee range is really far in this game, the fact that I can barely get away with 30%. Make it at least 40% if not 50%.
Concussion Missile should be an Instant cast.
Missile Blast is so weak for 25 heat! Reduce the heat cost or bump up the damage.
Arsenal- Isn't too bad if we gain more mobility it would make a huge difference, a slight damage increase overall would be helpful.
Pyro- Needs a lot of work, while it is enjoyable to play, the DoT's are weak and Heat management is awful.
Bodyguard- I have had nothing but success with healing on Merc so far, although It could use some love.
Overall I feel this class really isn't in an awful state, we are still very viable in all areas. Although if we gain mobility and utility, that would go a long way!
The trooper story is awful compared to other classes. After playing some other classes I really don't understand what the devs where thinking when the came up with the story. Smuggler is really good. I never got to far on mine but since the opening scene I keep thinking when I was going to get my ship back! Along with the sense of humor in the story I would suggest that as being probably one of the better stories.