Deathmatch is ideal for those players who chafe at the need to boost points or defend the Front. While Deathmatch isn't quite a free-for-all (it should properly be called Team Deathmatch), it does allow you to cut loose, and put your entire focus into killing your fellow man. You'll also get to use the power-up boosts you may have seen in Practice modes.
Deathmatch comes in two flavors- Elimination and Skirmish.
Skirmish is similar to Dominion in many ways- your entire team starts out in the same place, and your goal is to rack up enough points to 'break' the other team, at which point they no longer respawn and can be completely exterminated.
Key in skirmish is fighting together- a solid block of four players will chew up any team that tries to rush them one by one. Since there are no other objectives to go after, concentrate on supporting your teammates- every warrior that goes chasing after an enemy only to be ambushed is just giving points to your opponents.
That said, death isn't as punishing here as in elimination- you will still repawn in due course, so even a total party kill isn't the end of the match- just be a little more cautious in your next assault.
Elimination, on the other hand, is more like Brawl. All players spawn in close proximity to one of their adversaries, and their choice is to go for the duel, or run off to join forces with an ally.
As in brawl, your own speed and duelling competence plays a role here. Unlike Skirmish, you will not automatically respawn, but you can be revived.
Reviving downed teammates is a powerful tactic- essentially erasing enemy kills, so it's wise to keep an eye on those skull icons in your minimap. The round is won when one team has been killed off, and the match is best of three. As is so often the case, the ideal strategy is to get close to your allies and form a killsquad that can overwhelm any lesser number of opponents
If you have any doubt in your duelling skills, best bet is to run to your nearest ally. You almost always get a free hit at their opponent's back, or, if they've had the same thought as you, a chance to ambush their pursuer, who you might be better suited to defeat than your original partner. It opens up options, essentially, and the more options you control, the better your chance of winning a match.
Also, no kidding about guarding fallen bodies- an enemy unrevived is an enemy twice-slain. If your group is impatient to move on, but you think the threat is high, it might even be worth standing guard alone. Even if you fall to the enemy reviver, with any luck you can stall them long enough for your team to hustle back, smash them, and revive you.