PoK has almost every vendor-sold tradeskill supply, and at least two of each tradeskilling combiner. (pottery wheel, oven, forge, etc) so if you're in to tradeskills, PoK is a godsend, more or less. Can get pretty much any non-farmed item, without leaving the zone.
There's a huge amount of quests in there, for various levels as far as I know; one of them I heard about was a 'shopping list' which with selo's, could probably be done fairly fast. run around getting bits and pieces. :)
Of course there's the stones/books for travel, the bank very handily in there, and the library itself. about 90% of spells and songs are sold here, almost everything under level 60, at least. Some research spells that aren't, and apparently songs sold in HHK and Erudin aren't available, either. But still, it is a great place. :) it's starting to make nexus fairly dead, now.
People sell KEI in PoK, there's druid and wizard ports to it, the stones to pretty much everywhere in the world... it is a hub of travel. Nexus is kind of redundant unless you are actually travelling in to SH or Neth, or of course the bazaar. Who'd want to waste time porting in to nexus, just to run to PoK to go use a book to get somewhere else, when you can get a port/punt straight in to PoK, and go from there? :)
Anyways.. it's late, I'm rambling. I'll sleep now.