1. First, marines can't take much for 500 points. My list consisted of two tactical squads, a librarian and a whirlwind. I genuinely felt lost without my centurions! Similarly the Death Guard army that participated was even smaller, just a lord, a solitary marine squad, a helbrute and a cultist unit.
2. Guard can take a lot more. Two guard armies took part, one relatively elite with two veteran squads, 3 ogryns, 4 ratlings and an hq in a chimera, the other with about three veteran squads, a master of ordnance and a leman Russ punisher.
3. Bodies count, as the larger of the two guard armies was really tough to put down, few 500pt armies can muster sufficient firepower to seriously hurt that many troops, even squishy ones. Conversely, it didn't take much to hamper the more elite armies there as even a few failed saving throws made a big difference.
4. Don't be too hasty, every kill counts! The ratlings infiltrated onto the objective in game one, and died, the armies that were most successful marshalled their forces and timed their runs to the objectives perfectly. Conversely, don't leave it too late, as a single failed roll can ruin your day, you want to be hitting the objectives the turn before the game ends.
5. Some things look dangerous, some things are - they aren't always the same things! I had endless problems dealing with a single chimera, it's multi laser, heavy bolter and heavy stubber being a constant source of pain and casualties throughout the first two games, particularly since my multi melta was limited to snap shots when it moved. Similarly my whirlwind was a target of fear from all my opponents bar one, and it attracted a lot of fire.
6. Don't forget the rules. A couple of seemingly minor mistakes cost me dearly, such as look out sir on the librarian, who could certainly have tied up the ogryns in game one to give me a better finish, and making sure my whirlwind was hull down on the Tau too, instead of presenting a full side-on shot in the open.