<< Any hints for a newbie?  >>
*Don't try to play it like starcraft, it ain't about giant masses of units or a huge economy. A small group (like 10-12) of mixed units with different abilities can often overcome larger forces comprised of 1 or 2 unit types.
*Selective targeting. This should be a given if you've played about any RTS. You don't want your units just firing at whoever, you want them to all concentrate on 1 unit at a time (the opponents hero first unless he's running around TRYING to draw your fire). Queue them up even with shift+right click.
*Get out a couple units ASAP and go level your hero (notice I say hero and not heroes, don't build all three! don't even build 2 at the very beginning because the exp split makes them level really slow) on some creeps
*Use your Town Portal! I see so many newbies overlooking this and it is important. When your base is attacked while you are away your hero can use his town portal to get the troops back to defend. It can and should also be used to save your allies base in team games. 
*In team games don't drop immediately if you get wiped out, you can still help your ally micromanage his troops, which can be a big help
*In team games if you ally drops you can control all of his stuff, buildings units and all. Do so!