To give you an idea of the NUMBER of quests (these don't include the
required Missions), check here.   SPOILERS IN LINK.

http://gw.gamewikis.org/wiki/Quests_%28Nightfall%29

Yeah, massive improvement over Factions.

Heros and hero controls are one of THE best improvements in the entire game.
They offer TONS of flexibility and allow all sorts of hench possibilites
that didn't exist before. Now if I need a tank, I can load Koss down with
Gladiator's Defense, Watch Yourself, Bonetti's Defense, Dolyak Signet and
watch him shrug off damage like it's nothing. If there's an imporant NPC(or
tank) I need to keep alive, I can set one of the monk heros up as a
Bonder/Prot monk and bond the NPC to give them incredible survivability. And
if I'm going in a heavily populated area, I can bring along Olias and set
him up as a minion master to help even in the odds. They've already turned
the tide in several battles, during one, my paragon was faced with a monk
boss whose healing was simply too much for me and Koss to outdamage...until
Koss disabled the boss' Orison of Healing with Disrupting Chop, then
interrupted the boss' Healing Breeze with Distracting Blow. Not to mention,
by having access to EVERY skill you've unlocked on your account, you can get
some VERY potent skill setups early on(my ranger hero went out packing
Barrage, Savage and Distracting Shot, Screaming Shot, Marauder's Shot, Throw
Dirt, Whirling Defense, and a rez skill, all at level 11, my own ranger was
way past level 20 by the time he had all those skills). My only complaint
about them is now I gotta take the time to get weapons and runes for all the
heros I use regularly, heh.

Yeah, I took my ranger in there just to test and I was totally surprised at
the sheer number of forces involved in the battle.

No kidding. Just when I think I've completed every quest on the starter
island, I go back to some other location that had no quests for me
originally, and find that the last quest I finished has now opened up a new
quest or two.

Yeah, I got quests all over the place...I like that they have you go
backwards as well as forwards as well, no more "finish every quest in city,
never have to return to city again" feelings.

Yeah, there's some nice new stuff all around.

Another improvement I like is the ability to save your builds, and adjust
your secondary profession at any town. My ranger has PvE setups for
Barrage/Pet(R/Mo), Barrage/Interrupt(R/Mo), Dedicated Interrupter(R/Mo),
Hammer Ranger(R/W), Runner(R/Mo), Vigorous Barrage farmer(R/Mo), Touch
Ranger(R/N), Spear Thrower(R/P), and others, not to mention a couple of
alternate versions of some of the above builds for using in Alliance
Battles/Fort Aspenwood, and changing between them used to be a chore. Had to
map to Senji's Corner/Crystal Desert, change professions, adjust attributes,
reset skill bar, change weapons, and map back to original location just to
switch from say interrupter to toucher. Now I just hit "load template",
select "PvE Toucher", and voila, my secondary is automatically changed, my
attributes are set to the correct levels, my skillbar is loaded with the
toucher skills instead of interrupts, and then all I gotta do is swap
weapons in my stash and I'm good to go. So convenient, especially for
characters who have a wide variety of builds.

I've noticed a bit more lag in some places, but mostly runs pretty smoothly
for me.