The one problem I have with Source Engine games is their tendency to have the combat run and gun, most noticeably 'Half Life', 'Counter Strike' and 'Left 4 Dead'. Now don't get me wrong, these are great games (except Counter Strike for the simple reason, it's shit), but I grow frustrated when I cant aim down the Iron Sights to engage far off targets or ask my followers to give me cover fire while I flank.
In L4D teamwork is top notch and the 'infected human' kicks the 'zombies' ass any day, but when I see a 'smoker' stalking in the distance my finger instinctively clicks the right mouse button to aim down the sights and cap dat foo' . Bit to my distaste I only end up rifle butting my buddy in the back of the head. The fact that this only happens once every couple of months means it's fine and I can ignore it, also the fact that the infected are up close to you trying the pummel you to death before you can say 'OMGWTFBBQ!!!!one1' makes it ok. (the addition of a scoped rifle makes me forget altogether)
In Half Life 2 the atmosphere is perfect for a decent shooter but instead they picked a run and gun puzzle game. When I play it I want to be the combine soldiers who look kick ass and tactical but instead I have to play as this stupid physicist twat, Gordon Freetard, while killing uncountable numbers of Force Reconnaissance marines, aliens, zombies, CPs and Bio Engineered soldiers, still hasn't figured out that aiming down the sights increases his accuracy. Maybe that's why the rebels keep saying 'reload doctor freetard'......god I hate when they say that.
Counter Strike is for noobs, leave it at that.
Two mods have taken the brilliant source engine and have developed exellent shooters: SMOD:Tactical and Insurgency. Both of which take skill to play (unlike gayass Counter Strike). Insurgency's weapons are perfect to the detail (unlike gayass Counter Strike) and SMOD:Tactical's bullet physics are Matrix awesome (unlike gayass Counter Strike).
Having said all that, my favourite mod is 'Minerva:Metastasis' which is run and gun but still awesome.
( I Know I Know Shut up)