I find uniforms to usually be more rugged than regular clothing, so it can take more of a beating in the field; they won't tear as easily, and I don't mind getting them dirty. As for footwear, comboots are not always great. I prefer to have a light, breathable shoe...a Jungle Boot is alright for this, but you can find some lighter hiking shoes which do just as well (although they can get priceyer than army boots).
I find sidearms to be pretty superfluous; there is rarely a reason to put your AEG away (if your AEG is small enough) to draw your pistol, unless you're planning on humiliating someone. Fact is, a collapisble stock is, imho, better than carrying a pistol and mags as a fallback for CQB. The extra weight isn't worth it.
As for first time mistakes gameplay wise, I'd say not moving is a big one. Not playing tight is another. There's no point in staying in one spot unless you've got the perfect ambush set up, and even then, you only stay long enough to get your kill. AEGs aren't generally accurate at long range, so you need to move to get your kill. As for not playing tight, I find most people, even people who've been playing a while, tend to stick their elbows out when they aim, or stick their knees out while crouching...