Losing the Green
Meanwhile Hal is working on his secret identity and getting lots of subplot set up for the big Millenium crossover. Everyone else turns up in time for the main event (2). By this point Englehart seems to be running out of steam on the title, or perhaps he doesn't like where it's going. Either way, his writing is lacklustre and missing any subtext. Characterisation is practically at the level of everyone reciting their catchphrases at least once an issue.
I can see where Englehart might be fed up. It's clear that the entire storyline post-Millenium is a set up to destroy the team he's been building for the previous two years. This culminates in the trial of Sinestro where the Green Lantern Corps decides that since Sinestro always escapes from prison and goes on to commit genocide, the only option left is to execute him (3).
So they all zap him with their rings and he drops dead and the main power battery explodes. It seems that the guardians had programmed in a failsafe to stop them ever killing a male of Sinestro's race. Killing of any other race or sex is apparently fine, but they didn't like the males so they fixed it so they wouldn't be tempted to kill them (4). And just to make sure they were serious about it, not only would this make the main power battery implode, but it would turn OA into a black hole that would engulf the universe.
Seems like a touch of overkill, there.
And you know what? They totally forgot to mention this to anyone before they left. Isn't it always the same? You go away and there's always something. If it's not forgetting to cancel the papers, it's leaving a bomb that could destroy the universe on a hair trigger and not bothering to mention it to anyone.
Anyway, Hal Jordon saves the day, of course, because it's always about Hal Jordon, and all but a handful of Green Lanterns are depowered. What was an ensemble cast of equals is now Hal Jordon and his cheerleaders. And the comic is canceled so that Green Lantern can move into the experimental weekly anthology version of Action Comics that nobody liked much because it wasn't very good.
It's a contrived and sucky end to a good period in Green Lantern, where the focus was on the corps rather than a single ring weilder. Why DC felt the need to dismantle it, I don't know. My only guess is that sales were poor. It's not the only reason for canceling a comic or radically changing its direction, but it is the main one.
2. which I didn't have available, so had to do without. Thankfully
3. Which seems a bit harsh. I mean ok, Sinestro has warranted execution for some time, but they have changed their policy at this point not because he has done anything especially nasty but because their security isn't doing its job effectively.
4. which shows a lack of imagination on their part, if you ask me.