I can't answer because I don't know how much your guild means to you. I've seen plenty of players come and go from my guild. Some don't regret leaving. We weren't able to run all the raids in The Burning Crusade. My guild skews casual and we just couldn't get a dedicated team of 25 people to make the necessary progress into Tier 6. So some players left and they are happy where they wound up.
Others quit the guild and very much regretted it. They came to realize that the content they were able to see wasn't worth giving up the special type of social atmosphere that they left behind. Now, given the option to run raids with only 10 players, we can see (and have seen) all the content in Wrath. It's really been a win-win scenario for us. We can stay mostly casual and still run everything.
So it's easy for me to tell you to stay patient and wait it out. It's easy for me to say that you'll get there eventually, that Blizzard makes new content at a glacial pace, and that you'll definitely clear the first round of raids before anything else major gets patched in. Odds are that you will if your guild really wants to.
It's also easy for me to say, "Go forth and PUG." It's easy to tell you that PUGs have been more successful in this expansion than at any other time in the game's history. PUGs on my server have run Obsidian Sanctum and Naxx successfully. You just don't need that intense, guild-level coordination for most current raids. My guild doesn't even schedule Archavon runs anymore partly because it's so easy to PUG it (also because it's so difficult to schedule).
It's easy for me to say all that, because I didn't have to do it myself. However, I can imagine the frustration of being among the first in a guild to hit 80 and wanting to experience what the expansion has to offer, while having to wait week after week for your guild to catch up.
But if you're really thinking about leaving a guild just because you have to PUG some raids or Heroics for a while, I think you've got other issues with your guild. You can't treat this situation purely as a matter of seeing content. You have to ask yourself whether you really value being in your guild or not. Do you have good friends there? Do you have fun playing with your fellow guild members, regardless of what you're actually doing? Are you an important and respected member?
If you're answering yes to these questions, then I do urge you to be patient, even if it's easy for me to say. You have too much to lose if you give up on your guild now. Finish the excellent quest lines in Storm Peaks and Icecrown that you probably didn't need to ding 80. Go back and do those group quests you skipped -- and help some of your leveling members do them at the same time. Work on your reputation, take Wintergrasp Fortress, or level an alt. Play through the Death Knight starting area. Even if you don't plan to play the class, you won't regret it. There's plenty to do to enjoy the game while you wait.
If, on the other hand, you can take or leave your guild, if it just doesn't mean that much to you for whatever reason, then you might as well say good-bye. But you shouldn't leave just because you want to see Malygos. You should leave so that, hopefully, you'll find a guild worth waiting for at level 90.