Thought I'd chime in w/ a some thoughts from someone's who's played way too much MP since the demo came out (and is currently mostly playing, winning, and farming golds, both w/ a decent network of "friends" added via Origin and in random pickup games).
There are definitely lots of bad players out there, but it's usually pretty easy to tell if you're joining a lobby that knows what they're doing (for bronze it doesn't matter, but if you want to win silver or gold w/ randoms, you probably have to be a bit pickier).
First, does anyone in the lobby use their mic? Unless you're playing w/ pros, w/o that, gold will be nearly impossible. It's also a good time to find out if there's someone really annoying on the mic too. You're going to be in a game for 15-30 minutes unless you wipe out...
Secondly, taking a look at the lobby details - what are the weapons loadouts? Are they appropriate/decent? If you see an adept or engineer w/ 2 heavy guns, you can pretty much guarantee that they don't know what they're doing. For laughs, I played a bronze round w/ a friend tonight and put my Black Widow and Crusader on for -200% cooldowns on my Asari Adept. Stasis cooldown was 33 seconds. w/ my normal +200% cooldown w/ my Carnifex, statis cooldown is 3.69s.
Lastly, and this is less important except with gold pickup games, what N7 level/class level are they? An N7 100+ w/ a level 1 class is probably fine if they have a sensible gear loadout - they've probably just promoted their character, but an N7+level < 5 means they've never even played/won a single match. They probably aren't familiar with the maps and other basic mechanics. Obviously, this is what bronze is for, but since the defaults are 'random' difficulty, a lot of new players get dropped straight into a gold lobbies. That probably won't be much fun for anyone (and they tend to quit out after a couple rounds, leaving you short a man for the rest of the match).
Keeping these things in mind, I strongly disagree w/ Jennacide's assertion that you need specific classes to beat gold. I've played and won w/ teams w/ pretty much any/every class from Asari Vanguards to Battlefield Soldiers. Moreover, the classes she picks as some of the "must haves" are arguable to wrong (ie, the Quarian Infiltrator is the worst Infiltrator IMO). ..
So now, a couple actual play tips/thoughts from my experiences, starting w/ the most basic things (since, as the stats corroborate my experience, most people are horrible):
Most players go about buying packs wrong. You should buy *only* recruit packs until you've maxed out your common weapons. If you don't, you increase your chance of "wasting" your chances w/ a more expensive pack. Weapons get more powerful and lighter as they level up. The common guns (Mantis, Avenger, Phalanx) are actually all very good weapons at higher levels and are easy to get to X. The additional bonus is that you'll build up your reserves of equipment. Also, lower packs have a small percentage of getting higher class items, so you may get lucky and get some "freebies". Having lots of spare medigel, surival packs, and rockets means that you won't be worried about using them when you need to in a match. You'll also have tons of equipment - go ahead and use all of them, you'll eventually have more than you know what to do with if you keep playing, and if you don't well, what use are they if you haven't used them? Either way.
IMO, having more than 1 weapon is almost always a waste. I've seen some people effectively do it, but switching guns takes a *long* time and the weight effect on cooldowns will lower your effectiveness for just about every class. It's worth switching back and forth with your weapons and checking the actual time differences in your cooldowns (available in the power details screen). One exception would be the SMG w/ weight reduction, which in theory should change your cooldowns by only a fraction of a second, but the weight reduction mod is currently bugged (wah wah).
No one's mentioned this in this thread so far, but IMO the Salarian Infiltrator is probably the strongest/most well-rounded character. Damage is through the roof w/ a Mantis X, Widow, or Black Widow (or you can go CQC w/ a shotgun). Properly specced, the Cloak/Energy Drain/Shoot will let you one-shot just about everything except elites (which don't take that much longer). Cloak is a godsend, letting you complete objectives, revive teammates, and generally get out of jams. It's the only one I've seen successfully solo gold (I'm sure it can/will be done w/ other classes, but it's certainly the easiest).
I'm not a super weapon elitist (use what you find fun if you're effective), but there are a few things mentioned that are worth following up on.
* The Falcon is by no means the best gun. It's buggy w/ lag (as is the Scorpion) and it's just been nerfed to oblivion anyway this week.
* Someone mentioned Paladin as the pistol to carry - I strongly disagree. I have a Paladin I (It's an N7 ultra-rare so unless you've won hundreds+ of gold matches you're statistically unlikely to get it much higher), and while it's more powerful than the Carnifex, it has way too few bullets (base capacity of 16) even w/ the ammo mod. I'm up to a Carnifex VII, so its damage is approaching the Paladin I's and it has 2x the capacity.
* Actually, same post mentions the use of N7 weapons. Again, not true. Besides the Black Widow, every other weapon class has other guns that *clearly* outclass the N7s (I have almost all of them currently: Saber III, Wraith II, Paladin I, Talon I, and Black Widow I) and most golds are won w/ Widows, Carnifexes, Plasma Shotguns/Spike Throwers, etc.
While I'm on that post, and moving onto powers, that line about "Stasis, Sabotage, and Energy Drain" missed the mark too. Sabotage has been nerfed and was only good (broken!) on Geth anyway. Stasis has had a minor nerf and is definitely still good, but on Gold you're dealing mostly with almost all armored waves in the later rounds, which aren't affected by stasis at all. It's too bad most people don't know how to combo biotic explosions correctly. A double detonate adept (or better, a pair) are a sight to behold. The fastest way to clear Reaper Gold (there's a 17:21min full clear posted on BSN/YouTube) is w/ 4 adepts (2 Asari, 2 Drell was what was used, although you could just as easily sub in a correctly spec'd Human Sentinel). So that's Warp, Reave, Cluster Grenade added to the mix. Decoy on the Salarian Engineer does awesome things if used properly. Overload w/ double-chain is fantastic for mob CC. Heck, even the soldier can be effective (Adrenaline Rush to get an instant reload on your Widow!).
Oh, and on the classes - I'm not the biggest Sentinel fan, but lots of people love/are great w/ them. The Turian Sentinel w/ weapon stability is one of the few that classes that can make the Revenant somewhat useful. Tech Shield+Fitness synergize well for durability, and w/ warp and throw, the Human Sentinel makes for a nice less squishy adept that can self-combo biotic explosions. (Jeez, has Jennacide actually played MP? like every single thing she's said is flat out wrong).
Well, that was longer than I thought. For those that are interested, there's a pretty lively MP discussion at Bioware's Forums (BSN) w/ a fair amount of strategy/build discussions, etc.