I don't believe you can extend the trial as the trials are extremely limited anyways. There is a level cap, a chat channel limit, trades are blocked, mail is blocked, and I dont know what else. I would advise you to get the real deal soon if you like it.
Also, remember that the first levels deffinetly dont represent the whole game. You wont get many of the key abilities until levels that are multiples of 10. Shamans get ghost wolf at 20, druids get travel form at 30, hunters get it at 20 I think, every class gets a mount at 40 (you have to buy it or a few have quests to get a mount). There are tons of other abilities you gain as you level, those just stick out the most as travelling is a big part of the game. The faster you travel, the faster you can get things done and level up.
We have a few WoW threads already you may want to skim through.
has some good new player info.
also is a great resource.