Traits - Class
When players talk about class traits in game, they rarely use the proper names of "The Deadly Storm," "The Beserker," or "The Martial Champion." Instead, they talk about "yellow line" or "blue line" traits, indicated by the trait's color border. The only class traits I always have slotted are the red line Vicious Strikes and Deadly Strikes traits (which increase my single target DPS critical hits) and the yellow line Fervent Rage trait (Battle Frenzy fills up 5 fervour instead of 3).
What you should trait from there depends on what role you want to play: single target, AoE, or off-tanking. Slotting two or more traits from the same line grants a bonus. Slotting five traits from the same line unlocks a legendary skill (you still need to earn the legendary trait before you can use the skill, however).
If you're off-tanking, make sure to slot the heavy shield trait. While you can use a normal shield, a heavy shield provides a substantial defensive buff and armor increase.
On a side note, class traits are the easiest way to increase LotRO champion stats because you earn these traits as you use your skills. You don't have to go out of your way at all to earn them.
Single Target DPS
Red Line - The Beserker
2 Traits: Improved Criticals for Brutal and Feral (Savage) Strikes
3 Traits: Improved Criticals for WIld Attack
4 Traits: Fervour generates 1 pip every 4 seconds instead of 5
Legendary: Continuous Blood Rage (upgrades Blood Rage)
Yellow Line - The Deadly Storm
2 Traits: Increased AoE hit chance
3 Traits: -10% AoE power cost, +5% AoE damage
4 Traits: Reduced enemy parry/block chance
Legendary: Deathstorm (upgrades Great Cleave)
Blue LIne - The Martial Champion
2 Traits: Champion's Challenge builds threat
3 Traits: Sometimes, using Blocking Blades and Second Wind won't use a defeat response, allowing you to use more than one
4 Traits: +750 max morale while in Glory
Legendary: Invincible (upgrades Adamant)