However, I think the biggest mistakes that were made is what was stated above: 1) Not at all focusing on the game that is still out there, and 2), not telling us any information about what is to come in the next game.
When I play Guild Wars, I basically feel like I'm blindly playing the game. There is little to no content updates anymore except for rehashed quests (like the Z-quests and find Nicholas' crap) and more ways to title grind. We're all just being told to fill fill fill the Hall of Monuments, but have NO idea what the Hall of Monuments even does or why we're doing it (except that it'll supposedly have some kind of bonus for GW2). It's basically like ArenaNET screaming, "Don't ask questions! Just do it!" and expecting everyone to continue playing the game like mindless drones for the next...X amount of years.
If you're not going to give us any more content except grind, at least give us some sort of incentive on why we're even doing it and give us some kind of goal in sight. Tell us about GW2. It doesn't have to be the entire synopsis or something; just make us excited to work towards filling the HoM. Give us some kind of objective to be filled instead of trusting us to faithfully keep playing a dying game we don't even have an idea what it's going towards. Just saying that "It's a bonus!" is just so darn ambiguous, I don't even care. Give us some more screenshots, a little snippet of lore, information on game mechanics--something. I can see why people think this game is vapourware; they just suddenly dropped the information-flow like it was hot.
And if you're not going to tell us about GW2, at least give us a reason to ride the old one out to even go to the new game (if this is their plan. For all I know, maybe they don't care if the playerbase leaves, as long as they come back for the next game). Give us some content updates that isn't just a bloody carrot. How about new quests? Or throw in a mission somewhere or maybe something in the style of the Titan quests? Heck, you can even make some quests that'll tie in to GW2 or something and give us some new lore to look at. I think at this point, I'd even take another copypasta dungeon as long as it's not another "Here, redo these same quests over and over again so you can get something pointless like 3582905382529-coin storage bag or a moss spider! YAY" update. But with no new content, no goals in sight, and no information on the new game, I'm with Upier. It's harder to keep my interest here every day. Which makes me sad, because I love GW1. It's sad how they're going about this, IMO. They must have some REAL loyal followers.