Shaun of the Dead and Hot Fuzz are very distinct and clear in their inspiration. Dead being an love letter to horror movies, and Hot Fuzz for action. The World's End, when I saw it, didn't have an obvious inspiration or homage aspect. That's fine, but it's straying away from what the other two are so good at. If there is a clear inspiration enlighten me I may have missed it entirely! (Apart from the body snatchers later on)
Then there's the plot. At first it appears like it's going to be a life lesson for Simon Pegg's character in the midst of a wild night. I loved the role reversal in it between Frost and Pegg by the way. Then it plays on the fact that their lovable hometown has changed, a nod to how times change and possibly poking fun at nostalgia. That's fine and good, and you have the golden mile. Then it breaks down to basically what happened in Hot Fuzz, but with barely any development of characters. In Hot Fuzz, you get introductions to all sorts of characters and have an idea of how the quaint town operates. In this, it's just sort of told and just trust us this is weird because obviously it is they're robots look blue guts!
So then what does the movie become about? Fighting off these aliens I guess. Sure whatever, that happens, then what? A funny exchange and then all of a sudden Simon Pegg's character continues to have no development and he's actually bad ass again.
Sweet.