The top comment is a meme, but the free trial indeed is not time limited. It's kind of like a "free to play" version of the game.

Most of the limitations are purely social - can't send friend requests, party invites, or trades and similar. (There's a full list somewhere) But, you're still on the same servers, you can still use matchmaking, party finder, and paid accounts can send friend request and party invite to you.

What is there is going to be basically 100% of the story, side quests, and raids up to the level limit. And it will all be completable through the various matchmaking systems or NPC parties (where applicable).

It's just a mild bit harder than the regular game (since, as an MMO, it is a social game) - and much of the issues you can "solve" if you know someone that has a paid copy, or make friends in game. For the party invite specifically, I know I've seen requests go by occasionally for adding two free trials to a party - which the paid account can make and doesn't have to stay. In my free company (guild) there's a few random free trial players we've "adopted" and have in the chat linkshell, but who can't accept invite fully.

Source: I've played on both free trial to level 50 and paid to level 90.