Welcome mate! Can't say i remember you though.
SM - as always, tough, more expensive than most, great tactility. They can loadout to destroy any opposing list, but if you specialize too much you risk being overwhelmed by certain lists. Chapter masters unlock cool abilities and lists.
BA - same as SM, but faster and with less versatility. They are the fastest army for their toughness, yes including DA as all their tanks are fast. They have great shooting power despite their combat-oriented feel, and are my personal favorite army. I've played them for years (10 now I think?) and have ~8k points.
SW - same as SM but much more combat oriented. Some very cool unique units, the only SM army that can take beasts I believe. Fantastic models and outstanding versatility. Their devestators can split fire, but they (as far as I know) don't have access to fliers.
DA - as the newest SM codex they are extremely strong now. Cheaper Tactical marines, the ability to take bikes as troops, terminators too. Some cool new fliers, other units too. They are the way to go if you like bikes.
BT - old codex, fairly outdated I don't know much about it but I would hold off on getting them if you're thinking about it.
Grey Knights - you seem to know them, but they are the best of the best of the best. With storm bolters and power swords as standard, and the ability to increase the strength of their guns for a 20 points upgrade - they are a very formidable foe. The tradeoff is that there aren't many of them, but it's normally enough. Dreadknights are fun and deadly, but easily killed by termis etc. So - expensive but elite.
Eldar - they are expensive, pretty good, but also fairly fragile. Most units are overpriced, but there are still some great builds. They have the ability to take jet bikes as troops, have some fantastic eldar warriors ranging from the super-tough Wraithguard to the fast and fairly survivable but super-expensive jet bikes and shining spears. They also have access to MC which is nice. They are generally overpriced, I would wait until the new codex comes out for sure.
Dark Eldar - a glass hammer in every sense. Cheaper than Eldar, and generally more competitive, they are the fastest army by far (except maybe their fairer cousins). They revolve around speed and lightning strikes to achieve their goals. They have good combat units, a boatload of poisoned shots (gives MC armies a tough time, I can tell you) and dark lances to deal with any vehicles. A good army, but hard to use.
Tyranids - they have no invuln saves. Except the swarmlord in combat.... With swarms, MCs, and lots of great units this is a fun codex to play. They struggle with lots of flamers and high strength weapons (ie guard) but they are great against elite armies.
Chaos SM - they have lots of cheap MEQ, as well as cheap special weapons. Their bikers are phenomenal, and helldrakes are considered op by many. I am not one of those, but they are great at clearing infantry.
Chaos Demons - I don't know the new edition as well, but their troops in general are cheap and numerous, as well as pretty good. They rely a lot on psychic powers, more than almost anyone, and have lots of MCs. And hordes. And that nasty warp storm table. They're weird man..
Sisters - Meh. Lots of meltas and flamers, crappy models. Not worth it just for that. (I feel like Zion is gonna swoop in here).
Orcs - boyz. Lots of boyz. And weird tech. You get a lot for a little, and some great special weapons too - just remember to get the hell into combat. Their BS is 2 almost around the board. But, there are a lot of them.
Necrons - aka vehicle bane. All their troops glance vehicles on 6s. All the time. With the new rules, that's deadly. They have lots of fliers and can dominate you from there, and many MEQ units as well. A tough foe to face and a funny, themed army to play.
Guard - lots of men, lots of tanks, lots of fliers. A good army to play, by some considered the best codex. I myself have about a thousand points and I haven't lost yet. Leman Russ tanks are fun at AV14, and vendettas are considered over powered (though I doubt it). Great variety of builds, from full on mech to hordes and everything in between.
Did I miss anyone? I feel like I did.
If you have any questions, feel free to PM me. Or if you'd like a more in-depth overview of an army.
If you go down to the woods today, be wary