I wrote one very long defence of the film that addresses similar concerns to yours in
this thread, but I'll address this post anyway.
I just don't want to spoil anything, but I feel like a lot of the action is extremely weak and the ending is awful taking a weird post apocalyptic turn.
I guess that's your opinion, but I liked how well-choreographed and clearly-depicted the action scenes were. They were intense without being incoherently shot or edited without losing any sense of humour and the effects work was well-done. Also, the post-apocalyptic ending makes some sense given how the other Cornetto films ended in the wake of smaller apocalypses -
Shaun of the Dead end up with England recovering from its zombie epidemic, while
Hot Fuzz saw the village still running even after all the most important citizens (doctor, priest, police chief, etc.) were arrested. In that context,
The World's End is similar in how it involves humanity rebuilding itself after civilisation collapses.
We never really understand the reasons for these robots to be taking over the world and the ending of them just giving up is just so ridiculous.
Uh, there was that entire scene where the Network (the massive beam of light alien) appeared and explained why they were taking over the world. To recap: the aliens/robots are moving from planet to planet and converting the native populations to their regime. They claim to be doing it in order to improve the planets and join them together in a single peaceful galactic community, but they still come across as power-hungry monsters who see no problem with committing mass genocide.
As for the ending where they just give up and leave, well, it's meant to be a little ridiculous. This is a comedy, after all. The idea that an omnipotent alien intelligence can still be out-argued by a trio of drunks is a fundamentally silly one, but it also makes sense since the aliens realise that it's too much trouble to try to convert such a difficult species (hence why the alien's last words before disappearing are just, "f*ck it").
There isn't really a satisfying ending to anything. Pegg being a victim of addiction and depression didn't really seem very original to me; I have seen this dynamic in so many films throughout the years that it felt very done. It would work if it was a small story, but it kind of dominates the story of the film and it wasn't that interesting.
How much originality do you expect from the other Cornetto films, though?
Shaun of the Dead was about an aimless slacker finally learning to take responsibility and protect his loved ones while
Hot Fuzz was about an emotionally stunted workaholic learning to take it easy and open up to people. At least having his character be a mentally unstable addict lends some dramatic weight to his goofy behaviour and provides good motivation for the plot to keep going even when common sense dictates that it shouldn't. Also, of course it dominates the story - it
is the story. If you don't find it interesting, then that's a shame.
It didn't make much sense to me that Frost would stick around this guy after something like 20 years of being a f*ck up, he has no motivations.
At the start of the film, it is clear that Frost has not seen Pegg in years and wants absolutely nothing to do with him (especially since they have some unresolved issues that are revealed late in the film), so Pegg has to guilt-trip Frost into going along with his plan by saying his mum died. Frost doesn't want to be on the crawl anyway and he tries to bail on it around the same time that the aliens are first discovered, at which point he's forced to keep going with it. It's also pretty clear that Frost still considers Pegg his friend and is mainly just disappointed by how Pegg has constantly let him down in the past - friendship does tend to be more emotional than logical, after all.
I also just think that the other characters side stories were just stupid and didn't really need to be in the film, especially the relationship with Sam side story.
It couldn't just be Pegg and Frost by themselves, and the extra characters do reflect other sides to Pegg's own narrative. Eddie Marsan's character is also like Pegg's in that his life is still controlled by his high-school experiences (though his childhood was about traumatic bullying instead of Pegg's being the coolest kid of high school). Paddy Considine's character is supposed to Pegg's romantic rival to their love triangle with Rosamund Pike's character is supposed to further demonstrate just how immature Pegg's character is. Meanwhile, Martin Freeman's character is admittedly a bit flat and doesn't go much further than his being an uptight tech-savvy yuppie who is uncomfortable with people hitting on his sister, though given what happens to him it makes sense that they don't really bother to give him much more personality than that. These don't have to be great characters, but they make for solid support and their arcs do tend to pay off (especially in the case of both Marsan and Considine).
About the checking boxes, I just thought all the character changes were so predictable and most of the film I just saw coming from a mile away, except the apocalypse.
Yeah, this is a side-effect of the Cornetto trilogy's whole thing where every line of dialogue can be interpreted as foreshadowing a later event, especially when the opening montage foreshadows the bulk of the film anyway. It may make the film kind of predictable, but it wouldn't be the same without it.
It also felt very obvious to me that they left a lot on the editing room floor, their should have been more of a conclusion to Pegg's turn, not just him going back to the pub in the end.
There was enough of a conclusion. The fact that Pegg walks into a pub and only orders water is supposed to be a sign that he's grown beyond his personal demons - compare that scene to earlier on when he mocked Frost for ordering water. Frost's narration did point out how Pegg had apparently learned to find more of a purpose in life than just getting trashed all the time. The whole reason he and his robot friends go in there is to start a fight with the anti-robot humans inside.
I don't know if I can convince you of anything else, but I just personally think the last 15 minutes is just so horrible, it's probably one of the worst endings to a film I have ever seen.
Well, it's not like you've said anything more substantial than "the ending was terrible and I hated it". In the thread I linked to, mack went into detail about why she didn't think it was that good an ending.