I decided to try out playing Marksman to really see what is wrong with the spec and possibly offer some input to Blizzard about what sort of changes need to happen to make it better.
First off, It's DPS isn't as bad as people make it out to be. It is lower than BM and SV but it certainly isn't unplayable. Being mainly Physical damage it also has the benefit of scaling better with weapon damage. Towards the end of my testing I started noticing that you do have to pay close attention to your Trinket ICDs, Scope and Synapse Springs in order to time it with AmoC, Rapid Fire and Stampede. I was seeing quite a significant damage increase when I manged to do this well.
If you want to see logs from any of the fights (LFR Full Clear) as Marksman check the WoL Rank section of my WoW-Heroes. Do not expect flawless gameplay. This was my first time playing Marksman since 4.2 and I'm not even 100% sure if I was doing everything correctly.
We have a pool of 100 Focus, Aimed shot costs 50 Focus. This made perfect sense back in Cataclysm where the opening rotation was Aimed > Steady > Steady with nothing else. Now during our 100-80% burn phase you WILL focus cap a lot, there is no way around it with all our new abilities like AmoC costing 60 Focus, Glaive Toss/Barrage costing 15/30 Focus and Dire Beast/Fervor restoring Focus. with Serpent Sting damage also receiving a buff not long ago we now have to keep that up during the opening which costs 45 Focus from Chimera Shot.
All these odd numbers and needing to double cast Steady Shot and Aimed Shot casting 50% of our Focus Pool will inevitably lead to a lot of Focus Capping. To Fix this Aimed Shot cost should be lowered to 40 (from 50), Chimera shot cost should be lowered to 40 (from 45) and the spec should be given a Passive that makes our maximum focus 120 (like BM has).
The opening part was really the only part where the rotation was uncomfortable and I felt needed a change. While the lower Focus costs and increased Focus pool will help our damage slightly it still won't quite be enough to make it competitive again. An easy was to increase MM Damage without making burst over the top through Aimed/Chimera shot or effecting other specs would be to increase Steady Shot damage. Not sure on a number and it would likely be something that would need proper testing on Blizzard's side to find out a good number to increase it by.