It's a new Chapter, you get new classes. I don't mind paying for that. It's paying a monthy fee for the same game with lesser service and customer support that bugs me. You still get free new content with GW. They added two new dungeons and the Sorrow's Furnace area, along with new quests for level 20s all for free.
Nightfall does feel like its coming out too close to Factions though, and I haven't even finished all of Factions. On the other hand, unless they get majorly nerfed from the preview event, the new classes seem to be a lot better than the two Factions offered.
The chapter updates don't piss me off, it's ANets policy on character slots. If you bought Factions and Prophecies, but didn't link the two games on one account then you got 8 character slots. If you link the two games to one account, thus making less of a mess for their own filing system and databanks, they give you 6 character slots. So they cheap you out of 2 character slots for no real reason, then ask you to pay $7 to buy the slots you would have gotten if you just didn't link the games. Why not just not link the games? Because then you don't get to have all the skills and items you've unlocked on the new game, and those things are extremely important for PVP. So what is their answer to this? Well now you can pay like $30 to have all skills unlocked without earning them. This is how they are being bastards. The line was 'only pay for major content' which I would say new classes and a new Chapter count as, but character slots and skills that are in the game anyways are not major new content at all.