If people read the FAQ's anet posted and kept up to date with the updates they always post you would know:
From the start they said every 9 months or so they would release an expantion with the same amount of content as the origional release.
In Factions that means:
Over 60 new areas
A bunch of new mission styles, PvP and Co-Op.
An alliance system.
Over 300 New skills in total for EXISTING and NEW professions.
Loads of new armour and weapons, as well as new faces and haistyles for core professions.
New pets.
The way it works is this... If you have GW and decide to not buy factions, you can't access Cantha, you can't play as an assasin or ritualist and you can't use any of the new skills they introduce for the new classes. If you wanted the new classes will still be able to use the new weapons like dual wield, even if you don't buy it, however you can't customise so damage would be limited.
If you don't have core and buy factions you can't access Tyria, but you will have access to all the core professions and skills as well as the new ones. However again you would be limited in the armours and items you can use as you would have to trade with someone who did have Core and Chapter 2.
If you have Both core and factions, you get everything! Loads of content, loads to do. It may seem that people buying the Factions set without core gets a lot more content than people who already have core, but if you think about what happens when chapter 3 gets released you'll understand how the chapter system works. Think about in a few years when 4-5 chapters are avaliable, then for someone coming into the frachise it's more a selection of what content do they want, which is how Anet want it
