If GW2 is going to be alt friendly or not is definitely a difficult question to answer.
On one side it has accountwide achievements and absence of mounts, so you don't have to miss anything (except pets maybe).
On the other side, earning achievements on alts is one of the things that made them a timesink in the first place.
One thing is very likely though: GW2 will have a high amount of replayabililty to it.
Personal stories, dynamic events, all those features are there to stimulate replayability.
But the question still remains: will you play it on an alt or stick with your main?
This is where GW2 scores a bit lower then other games.
Where in other games you needed an alt to replay lower lvl areas, you can now replay them on your main.
This discourages the creation of alts by default.
This, of course, doesn't count for personal story.
But since personal story will create a deeper bond between player and avatar, it might be harder to leave your main character for an alt.
Of course, it's not all bad.
Downscaling might discourage people to create alts, but it also makes alts less of a thing to plan in.
In WoW, you and your friend have to plan out times, builds, races and quests just to play together.
In GW2, friends can join and leave on the fly, making the planning less tedious.
This encourages people to make alts.
Also, while personal stories may bind you more to one character, playing several characters has the advantage of experiencing more personal stories and therefore more lore.
Since personal story also ties into race, playing more then one race is a great way to get to know a lot about the lore of every race.
All in all, It's very difficult to determine if GW2 is going to be alt friendly without playing it first (for a long period of time).
There are goods and there are bads on this front, so it'll probably depend on the person playing.
I will definitely make at least one alt, perhaps more