Tips for rogues:
Keep poison applied on your Backstab dagger. I see alot of rogues who only poison their usual weapons and forget about the dagger in their bags. It helps ALOT since if you need a waylay proc, chances are you're gonna need the extra poison damage too.
If you're a NE rogue, Shadowmeld is your friend. You can open up on someone with Ambush and gouge, shadowmeld stealth and ambush them again, and then finish with the 5 point evis. Also shadowmeld absorbs any attacks that are in the process of hitting you. For example, if a mage is casting frostbolt or a hunter just released his explosive/aimed shot, if you hit shadowmeld before the damage hits you, it will completely absorb it and any other effects.
If you are fighting against another melee, fighting your enemy so that you are always in the frontal position is MOST important. Most properly geared rogues have over 50% dodge chance and if you let a melee get even a second of opportunity behind you, it can mean your end. Usually when a rogue pops evasion, so does the other and the fight winds up being a fight over who can run behind each other to land a blow (since you both should have over 100% dodge chance).
If you're enemy is backstrafing, that CAN be to your advantage. This does allow your enemy to dodge your attacks, however it also allows you to gouge your enemy and then you can run behind them
Never, unless you need to stall them, open up on a decent hunter in any area without LoS if you're teammates are not around. Sure, rogues say save your shadowstep for when they disengage, but hunters have scatter shot too, which with these two getaways you will normally lose. It takes luck to beat a decent hunter (for example, getting some big crits on them; or if their scatter shot is on CD). Your best shot for taking down hunters would be to ambush and then immediately gouge and then backstab at the end of the gouge. They will proceed to use scatter shot/disengage, which you will need to shadowstep, sinister strike/backstab and 5 point evis for the kill (again, you need luck). As it was stated before, don't use sap on you're target unless it's necessary (if you're opening up on a BM hunter, sapping their pet is a must). Having your trinket up when solo'ing a hunter helps, thought it doesn't always assure that you'll win. These tips right here generally apply to mages also, though I feel they are easier to kill.
Also if you are playing O, and you are carrying out an assault on the EFC, do not die until that EFC is dead (even then you should try to stay alive haha). You are of no help to the offense when you are dead and sitting in the graveyard on the other side of the map.
Also if you are deciding which healer to sap, priests take priority over anyone else. Pesky shields, scream, and flash heals.
And one final tip: please do not GY farm (as in excessively killing out a team for several minutes, not just a few kills at the GY). Even when I am in the position to farm the EGY, I try not to. Those 5-man Waw Tawent pug-premades things that happen daily tend to do this, and if you're wondering how Pizza gets all those kills, it's because he GY farms ALOT. I frown upon Waw Tawent for this, and other guilds who do this. It discourages people from even playing in the bracket and I even notice that certain people (like pepperoni pizza) who GY farm others, /afk when they are being GY farmed. I reiterate myself: please do not GY farm the poor people