It makes people mad because we're all trying to make sure that the game gets salvaged. We're all playing, waiting with bated breath in hopes that SE will do something so that the game can at least be on the same level that XI was at launch in NA.
This last developer's letter, while it was ok from my standpoint, and hasn't really swayed my decision to keep playing, really upset a lot of people. The reason for that, is it's so damn vague....all of the letters are vague and full of promises (auto-attack, class gear, making actual jobs or job names like XI where they're distinct and give people a goal to strive for, etc), but we still haven't SEEN when any of those promises are going to be coming through.
As it stands, we're still sitting on a game with little to no content, with, in my opinion, a sub-standard battle system, horrible class system, and no airships, no chocobos, nothing that really makes the game stand-out as a true "Final Fantasy" title. To be honest, the only thing that makes it feel like FF is the Moogles....and even then they're rare to begin with. Fans and current players of this game, aren't so much upset about the fact that the updates are slow, or that the letters and posts aren't daily (from what I've seen)....they're pissed because the game just isn't a "Final Fantasy" title, and hell, it's actually kind of hard to classify it as anything right now to be honest.
Then too, you have the people pissed off with the updates, because certain things (quest icons, etc) make people feel like SE is trying to turn the game into WoW....I quit WoW, I quit Rift....if I wanted to play a game with a similar playstyle to those, then I'd still be playing either of them. People bought and are still buying XIV and XI because of the unique offerings that cater to those looking for something different in the usual MMO formula.
In short, people are just upset for varying reasons across the board, but ultimately the one thing we all have in common, is we want a next-gen FF MMO, and we want the game to at least succeed on some level. Unfortunately, the game is still steadily, and slowly dying, and I think we're all just a little bit afraid of it reaching the point where so many people have said screw it, that SE finally pulls the plug....because right now, at this time, this game is just a giant money pit for them, and they don't appear to really be focusing (even in the past, I'm aware of Japan's current events) on turning the game around in a timely fashion, and even the most die-hard of FF fanboys are giving up.
As an example, I started back up a week ago, and logged into my old character. Every single LS I was in before, is dead....one having over 90 members, and not a single person being on.....and that was on Wutai, which wasn't close to being a dead server before. I actually had to remake a new dude on Figaro in hopes of seeing some signs of life somewhere, and while there is more than Wutai, the game's population is a far-cry from what it was even 3 months after release.