This is an interesting story, because it feels for the first time like Doctor Who is trying to do "event" television. In a way, it feels very modern, sort of like a season premiere or a season finale on a modern show (and had they not held it back, it would have been). That's almost certainly part of why it's so beloved by fans; it's got the shocking return of the show's most popular enemies, it's got the show's first cast change...we fans tend to be suckers for big events that change the metastory, and this is the first big one. That's a lot of weight for it to carry, and it's no surprise that it frequently winds up being loved more in concept than in execution. It should be a big, epic event, but it doesn't fit into that box at all when you watch it.
It's not without its charms, though. "World's End" plays very well to the strengths of the classic series; the same slowness that people complain about when they watch the old Hartnell stories transforms here into a gradually building atmosphere of dread. This is a story that needs to seep in; giving the regulars a full episode to slowly recognize the creeping wrongness of the London they've arrived in greatly enhances the sense that this is a world where the Doctor and his companions simply should not be.
Every moment of the episode works to heighten that tension. Little details--the lack of sound, the decaying buildings and crumbling brickwork, the weeds growing up and out of every surface...it all leads to a sense that this is a London that's not just forgotten but abandoned. The poor costuming and bad effects work on the Robomen actually help to enhance the mood rather than looking silly or cheesy; they look like zombies, like the Daleks have put in the absolute minimum effort in creating their slave labor and don't care about their welfare at all. Tattered clothes? Wasted and wan flesh? As long as there's another human around to take their place, it's irrelevant. They're unbearably creepy, and the story uses them to good effect in this episode by showing them sparingly.
Everything works together to create an excellent atmosphere of looming dread. When the TARDIS is buried in rubble, it feels different than the usual "oh, this is how they keep them in the plot this week" plotting. It feels like they're trapped somewhere they don't belong. When the Doctor asks Ian if he's curious about what's happened, Ian's simple "No," is more effective than any speech. (Which is good, because as the rest of the episode shows, dialogue isn't Terry Nation's strong suit.)
And then the money shot. After a full episode of gradually building wrongness, of a slowly growing and utterly terrifying mystery, we get the Robomen en masse. We get the flying saucer. And then we see it, the first ever recurring enemy coming out of the river. This is the moment that cements the Daleks' place as the Doctor's arch-nemeses, even if it's so badly shot that everyone who remembers it remembers it wrongly (the Dalek begins its ascent from the river before the characters can react to it). The Daleks are back. Even now, over fifty years later, it still feels unimaginably huge.