While Marvel didn't invent the idea of a shared universe, they certainly turned that idea into a household one. From the moment Nick Fury showed up inside Tony Stark's home, it was clear the MCU was going to become a complex and connected world. However, some people are concerned that those connections have truly become convoluted, with recent movies like Captain America: Civil War and the upcoming Avengers: Infinity War making it clear that the days of the standalone hero adventure are numbered. This means someone who might have been a casual fan of one or two characters and only went to those particular movies is now required to have a working knowledge of dozens of heroes, villains, and ongoing plots, making the movies as indecipherable as their comic source material has sometimes been.
However, something even the haters have to admit is that the connections between characters and movies make it all that much stronger. You didn't have to be a Spider-Man superfan to realize his appearance in Civil War was a badass, crowd-pleasing moment. Similarly, you don't have to be a scholar of Tony Stark and his family to realize that all of the surprising places that his father Howard Stark pops up helps the universe feel cohesive.
Perhaps more importantly, it makes the universe feel lived-in and real. If supervillain threats regularly pop up (and the movies make it clear that they do), it would be weirder if none of these characters ever ran into each other or had connections to each other's world. So, while it may be fun to talk about the cluttered cluster of the MCU, we all like it from the bottom of our Arc reactor-powered hearts.