Guild Wars Upgrades

I currently own Guild Wars (Prophecies). How is Eye of the North (expansion) different from Factions or Nightfall (Full Games)? Is it that EN gives only new skills and quests, where F and N give new characters?

Also, I read that characters can be transfered (or rather expanded) from Prophecies to Factions. If I transfer a level 20 character from Prophecies to Factions, won't that make the quests easer than if I started right away in Factions? If I start a character in Factions, can that character go through the Prophecies quests (assuming I purchased both games)?

I already read the GW FAQ, but am still confused.

Eye of the North requires you to have one of the other games. It has exclusive level 20 content which means there are no new professions and you don't get 2 more character slots for purchasing it.

If you take a Prophecies character to Factions, the quests will not necessarily be easier because the pace in Factions is different. In Factions it is a lot easier and much faster to get to level 20, which means most of the game is made for level 20s.

Also, if you buy another game, be sure to ADD THE ACCESS KEY TO YOUR EXISTING ACCOUNT! It's bolded and in caps because a lot of people don't think to do this and end up with two different accounts, and once they're created Anet and NCSoft won't do anything for you.

Bssically yes, no new starting area, no new classes, no additional character slots and is an expansion and thus needs one of the 3 chapters to be able to play.
You're assuming that the characters will be transferred to the starting area. If they were thaen yes it would be easier, however they are transferred to the area where the level 20 content starts in Factions and in Nightfall, the area in Tyria (Prophecies) is a little lower than that, around level 16.
A character started in Factions cannot do quests prior to Lion's Arch, they can do the Missions and access all the zones, but not pre-searing obviously.

Basically, yes - as has been said - new areas to explore, too.

Characters are not "transferred " from one campaign to another. You simply have access to the other campaigns. Once you finish the simple quests required to get to the other campaign, you can use the "boat" on the map to travel freely between the campaigns.
If you do the quest that takes you to Factions, you will end up in Kaineng Center, which is past the early stages of Factions. You will be starting at stuff designed for lvl18-20 characters.
The Factions character would need to get to Kaineng Center first. There you can get a quest that takes you to Lion's Arch. You can do the quests from there on. I don't think you get offered any earlier quests (such as in Ascalon)

You are correct; you cannot get quests offered before Lion's Arch. However, you can still go back and do missions. But you won't receive the rewards for them in normal mode (but you'll still need to do them if you go for the Protector/Guardian of Tyria title).