I don't mind the quests being linear for the main story line. That is what I would expect. But, at the moment that is all we have to level through is the main story line. This gets boring quickly if you try and level more than one character. Below are my suggestions on how to improve on what Blizzard has already done.
1) They need is to add more random quests that require exploration to find and open up a whole line of quests that are completely optional but deal with the lore of the zone, or progress a story line that is just an offshoot. Every zone should have one or two optional story lines (in my opinion) in addition to normal story line that they have now.
2) They need to add more diversion in the story. Allow us to choose our path in the zone. Give us options, where if we pick one quest giver we cannot do the quests from someone else. Make the quests achieve the same goal, have the same number of missions, but require completely different tactics and items so there is variety in what we are doing.
3) Provide more Sandbox quests, mob dropped missions, world elites, and easter eggs in zones that encourage people to explore, kill random mobs, try new things, talk to NPCs and otherwise interact with the world. Add in mini raid bosses that need to be teamed up to defeat at level and phase them for only people on the quest (or in the team of the person on the quest). Not everything has to be soloable at current level unless it is part of the main story line.
4) Add quests that are tied to professions, like there used to be. Allow people to use their professions to accomplish quests and possibly get some decent, level appropriate, rewards. My Hunter is a Leatherworker and I remember there being quests in Feralas and in Azshara that were tied directly to being a Leatherworker. I also remember Blacksmith quests in Tanaris and in the Burning Stepps. Make having a profession meaningful as we level and not just something that gives us a stat bonus for raiding during end game.
5) Remove about half of the current flight paths that were added. I understand the need to have a few more, but I think Blizzard went waaaayyyyyyyy overboard in this dept. Each zone should have two, maybe three, flight paths depending on the size and layout. but, take Eastern Plaguelands as an example. There are like 8 flight paths in the zone. This is serious overkill and not needed. I just stopped picking up flights after a while because a lot of them were just not needed or wanted. I may be in the minority in this, but I really think there are too many now.
6) Finally, we need some mechanism that makes teaming at all levels of the game engaging, accessible and encouraged. Currently it is a single player game until you hit level cap because nothing requires a group and if you do group it is tedious, at best, to keep track of missions and completions of team members unless all you do is run instances. Look to City of Heroes as a good example of how to make teaming at all levels fun and rewarding. There has to be some way to make it better than it currently is, and I really hope that Blizzard takes the time to make it happen.
Anyway, those are my suggestions to improve the leveling process. I like what Blizzard has done so far, I just think it needs some tweaks and additions to really make it feel complete and offer variety to the leveling process.