Stasis no longer worksas well on Cerberus to the extent it once did with the addition of Dragoons, and the other 3 races have a ton of armored units.
Geth got an absolutely horrific buff, and are far and away the hardest faction to skilled players because of BS staggers. On the plus side, no Sync-kills whatsoever on Geth, so they are relatively easy on the lower difficulties. Still BS overall though. Collectors are genuinely difficult on pretty much all difficulties depending on your kits and weapon strengths. Cerberus are the second-easiest faction now instead of the weakest because their weakness (lack of coverbusting units) was plugged with Dragoons. Also, Phantoms are BS to the extreme, and now have a bubble they can deploy to nullify powers. Reapers are the easiest faction because their only coverbusting units are husks, which are pathetic, Brutes, which are easy to spot, and Banshees, which are their boss unit and are honestly not as scary as they look, especially since they had their coding nerfed to not sync-kill 24/7.
Top tier classes are as follows
*Literally every Infiltrator that isn't the base Human Infiltrator
*pretty much every Turian aside from the Turian Vanguard
*pretty much every N7 class
*pretty much every Drell except for the Drell Vanguard, and that's debatable
Every class in the game is still extremely potent on Gold, and most are still definitely Platinum Viable if you have good gear (which will take a loooooooong time)
Top tier guns are
The Cerberus Harrier (ABSOLUTELY THE BEST WEAPON IN THE GAME)
Claymore
Reegar Carbine
N7 Typhoon
Prothean Particle Rifle
Talon pistol
Black Widow
Collector Sniper Rifle
Do the objectives. Objectives are the only way to fail a match aside from a full party wipe, because of time limits. Concentrating on doing them efficiently also rewards you with more credits and exp, so that's good.
General tips
*If someone is shooting at something and that something isn't a boss unit or an extremely strong elite unit such as a Phantom, Don't bother shooting at it. There are more important things to shoot at elsewhere and that guy has probably got it covered.
*If you find yourself trapped in a cycle of deaths-revives-deaths between 2 or more players, do NOT be afraid to cobra missile the surrounding area. Missiles are a generally plentiful item and completing the mission my give you a minimum of 5 more if you buy any pack.
*Do not be afraid to Med-gel if you are in a position that is detrimental to your team to attempt to revive you, but at the same time, give your team time to revive you if at all possible. Wasting med-gel is silly, but never using it is just as bad.
*Timing Ops packs is an extremely beneficial skill to learn. Ops Packs give you 1.5 seconds of invincibility, cancel all dots on you such as Banshee warps and Atlas missiles, and restore both your shields and health to maximum. Proper use of ops packs essentially doubles your med-gel capacity.
*There is a maximum amount of enemies spawned at any one time on the level, which is 8. Once you remember this, if the situation gets hairy, you realise that running away is probably the best thing to do. You are more likely to survive if you walk into a spawn without realising if you just run for it, and there is unlikely to be anything more scary where you are running to. It's by no means a perfect rule, but it's a solid tactic to realise at least.
*Cooldown percentages lie to you because they do not scale linearly. For example, on a power with a large CD time, the difference between 100% CD and 150% CD can be up to 5 seconds or more on the longest powers. In contrast, the difference between 150% CD and 200% CD is usually closer to a second at best, and is often closer to .3 seconds or lower. If you have a kick ass awesomesauce gun that you want to run on your adept, for example, as long as the CD is ~170% or so the damage increase from your gun will more than make up for a .3 second recharge on Warp. Guns have the highest DPS in the game above all powers- remember that.
Reddit Mass effect coop has a ton of in-depth info with players that have played since the demo debuted all the way to the current day and is laid out wonderfully for new players to ease them into it. It is well worth the time to check out for more info.