Fast travel hinders game design. Hinders quests and quest direction.
How exactly?

Fast travel being in the game may encourage the designers to make quests that require large amounts of long-distance travel...
Indeed... and I'm sure some see nothing wrong with that at all.
Myself, I'd not mind a few of those... But if I did not want to take the time (either in FPP/realtime, or via Map-Travel), then I would not do that quest.
There's nothing wrong with fast travel. If you don't like it don't use it. Nobody is forcing you to do so.
I disagree. I see plenty wrong with [so called] Fast Travel. The main flaw is that without taking encounters into account ~it really does imply 'instant' (by-pass) teleportation ~despite the implied intent that it's an unattended forced march to the destination. In practice it's an exploit... They need to properly account for the implied travel time and apply this to any spell effects... So if you 'Fast Travel' with effected abilities (and are hauling 25 magic warhammers for sale in the city)... the spells should expire just minutes into the trip instead of after you arrive (days later) on the other side of the realm.
** Bonus points for implementing an 'in-between' encounter system that loads an area somewhere [appropriately] between points A & B ~to represent where an encounter occurred along the way.