I do not think fast travelling is bad.
Like others have pointed out, you'll get bored of "travelling the world" (asuming no mounts) over and over again. Now, do not take this the wrong way. At Baldur's Gate series I loved to explore every map, but when you need to get to point A to point B, across say, 10 other maps you visited and re-visited like 10 times on a quest, it was tedious and pointless to walk all over the place (no running in Baldur's Gate, heh).
As I see it, portals and fast travel are there for a good reason.
See WoW's flightpaths. Pointless to get on a "taxi" and wait for 10 minutes, but still is a "fast travel". So, in my opinion, GW2 nailed it.
I think you have to have both.
A fast travel system lets you...well, travel fast when you want/need to. Can you imagine how frustrating it would be to be on the other side of the world when an elder dragon champion popped out? o.0 And if all your happy-go-lucky friends were there and you weren't?
At other times you may want to skip and frolic through the countryside at your leisure picking daisies and discovering all manner of wonderful things.
They've said they want to cater for everyone. So...no mounts does that for some. You enter a zone with many DEs and run around doing and finding things. You want to go back to Divinity's Reach. Fine. Bam. There. That does it for others.
Just another example of Anet's Godlike ability to make everything brilliant in an all around way. Not that I'm a fanboy ofc.
n.b. I once ran from the top of the eastern continent in wow to the very bottom of stv. >.> I like exploring but it would suck without some way of getting around quickly too.