Ignore the previous poster, he is clueless about thanes in their current form.
You provide tons to the group as a thane, in the following roles: interrupter, backline peeler, guard, pet cleaner (!). Just don't try to play as an offensive shield tank, it won't work as thane, their damage and skill lever is much lower than warriors.
Interrupting is one of the most important (and underestimated) roles in this game and thanes are excel in it. Their quick nuke + insta dd makes them great at interrupting those pesky casters, but of course you need some experience and battle awareness to spot the dangerous caster quick. Backline peeler and guard needs no explanations: make sure you have 42 shield spec, and the skills Slam (shield) and Conquer (hammer) on your best bind keys: whenever an enemy runs towards your casters line, slam them or conquer them, switch to next target, rinse and repeat (and keep interrupting meanwhile.. (I know its not easy to do multiple tasks but you picked this class so you have to do it). Pet cleaner is also important: those casters who sends shitload of pets (like theurgs) you have your own instant pbaoe to kill them at once.
I think thane is one of the best solo roamers that has no speed, because of the ranged interrupts and slam in one package. I solo roam with warrior a lot and here is my strategy: I always set up camp at a keep Midgard owns inside enemy territory, get hastener+potion buffs+endo 3=perma sprint, hit targets (many times I can win 1v2 even, with slam only... and you have cool interrupts on top of it).. after a fight, I run back to keep, get hastener speed again, etcc. In 1v1, usually slam, sprint to the behind the enemy character model, push /stick (bind it to a 4th or 5th button on mouse), that will immediately turn your char to the enemy back, switch to 2h weapon and process your hardest hitting combo. It will give you nice advantage and you will probably win. Against casters its even easier for thanes, since they won't be able to cast much and you just keep running after them and dd+pbaeo on cooldown, the second they stop you are on them with melee and game over.
In keep fights is when thanes can actually lay back and releax, spamming aoe hammers and laugh. Need no explanation here I think
All in all, thanes play differently than warriors in group, but a bit more powerful than them in solo pvp (no difference if 8person group ganks you, which will happen lots, but its ok, you will have your glorious moments as well!). Hope this helped.