I'd just like to point out that your way of playing requires a balanced and skilled team which you aren't always rewarded with. At least in my experience. I like Medic so play that a bit, but I spend more time killing than healing simply because it saves my teammates more than healing would. Why heal them so they can swing a katana 50 times at a group of crawlers when I can shoot them and heal the Support who's actually doing something.
You don't always get an SS, usually don't get a Medic since people rant when they die. And if someone's playing stupid then I ain't healing them. Long time pub player in quite a few games, and it's best to play solo in a team. So by all means heal people as Medic, but do your fair share of killing. Light some zeds up on West London as a Firebug, but know that someone is going to come along and shoot them so you're better off killing them.
I agree, don't weld doors as much, light zeds instead of going for hs's with the Mac10, heal instead of shooting as a Medic. In a perfect world where teammates appreciate the healing and give you money for the crossbow, where they don't shoot zeds at range that you've lit up and ignore the mob right up close.
Leave strategy for guides on how to play well in a good team, leave this for hints. Hints like reload, reload, reload. Headshots with everything bar flamethrower and chainsaw. Quick Iron Sights is more useful than Toggle Iron Sights. Scar on semi-automatic at range can quite easily function better than the M4 and take out the majority of lesser zeds like clots, crawlers, gorefasts, husks. Fully auto only at close range against FP's, Patriarch, massive groups of mobs that you can't kill fast enough by going for the head.
If spraying on full auto, try crouching and aiming at the waist/chest, means all your bullets will hit. Even aiming at the legs works, recoil will bounce it anyway and as a bonus you'll hit the crawlers.
Don't snipe with the MP7M, wait till close range, quick iron sights and spray on full auto at head height. Can waste 15 clots in <2 seconds. When slo mo kicks in, aim your bullets at the head instead of spraying groups, you can hit, move the gun, hit and not lose bullets but hit each mob in the head.
Or summing up, reload and play everyhing as a Sharpshooter whilst keeping your heals up and moving around the map. Skirmish, don't entrench.