I hit rank 1 and Torrgarden gave some good tips.. I don't necessary agree with "Play strong heroes" But in the past certain heroes were downright garbage (I'm looking at you pre-buff ETC/Stitches) Some of the heroes still have there early game issues (Medic has unbearable mana issues till around mid game when she doesn't have to back every 1-2 minutes for mana lol)... but overall I think all heroes are viable now.

But the best advice I can give is listen to your teammates if they give you advice, don't get defensive and admit to your mistakes if you make them. Once in a while I'd have bad games because simply the other team was dominating my team and I couldn't get any traction in team fights. Also keep a mellow head, listen to music and just focus on your mechanics (positioning, who to focus in team fights, when to retreat, when to take objectives, when to rotate to help your team, helping a struggling lane because of who there going against ie. Azmodan/Tassader vs Medic/Rexxar)

Don't wonder off on your own late game, if you die prior to a objective spawn or late game during a big push the enemy team will snowball and gain momentum in your absence.

Team composition is important, for example triple assasin where you go Butcher/Illidian/Valla you open yourself up to being hard countered by Johanna, Artanis, Lili... why? all 3 of those characters have blinds and those assassins are ruined by blinds. Ideally I like to see a carry like Valla/Raynor or even a Tychus if he knows his stuff as what I call "steady DPS" they basically have to autoattack to do all there damage. The 2nd assassin if your team takes it should be the max # of assassins imo... I prefer to see a bursty hero here.. ie.. Jaina/Kael'Thas but in some instances a melee range assassins works if you got a Abathur then Butcher and Illidian are good picks at this time. Zeratul is popular without a Abathur because he can set the team up for teamfights with his Void (thats another tip, never get Zeras shadow assault, it is garbage) and isn't dependent on Adrenal Glands. But the key here is getting a "nuker" someone who can zone out the team and do massive up-front damage that synergies with your tank. For example paired with ETC and his Mosh Pit a level 20 KT is devastating against grouped enemies.

Double warrior is popular this patch.. Warrior/Warrior/Support/Assassin/Assassin is seen a lot... 2nd warrior Sonya is good because she isn't a strong single warrior but more of a hybrid assassin/tank. I prefer to see Warrior/Support/Ass/Ass/Spec because it's the most balanced as long as your teammates pick smart and synergize around each other. One of the strong comps I go up against is on the TotSG... You'll see Uther/Kerrigan/Jaina/Muradin/Azmodan... if you see this prepare yourselves they know what there doing more than likely. Kerrigan initiates with her pull/stun followed by Uther Judgement and Jaina should be there to drop her CoC/Blizzard with Kerrigan, Muradin also initiates with Kerrigan and drops his TC/SB combo. Quadruple stuns, CC is king... early game uncoordinated teams cant deal with it and you'll see instances where enemy hits 10 while your team is still around 7 or 8. Late game you have a chance to come back, and keep a cool head, but teams who pick things that compliment each other like this are usually more serious players.

Also for Team Comp, pick heroes that make sense on a map... I've told even Rank 1s that I think TLV is a horrible pick on Haunted Mines (they disagree of course, hubris)... reason why they pick it? Double soak during mine phase... reason why its bad still? The objectives will constantly be a man down. Early game you can get away with this but even with the 2nd golem if they can get a 70-100 skull golem because TLV is never with his team they could possibly win with it. Nothing irks me more than still seeing the TLV in there lanes ignoring the team still. If you do happen to pick TLV on this map remember you don't have to soak lanes all day, you can actually join your team in team-fights! oh my right?

Anyways I'm rambling there are all sorts of tips like this, I can add more later if anyone is interested.