The major things we saw wrong with Unix when we started talking about what would become Plan 9, back around 1985, all stemmed from the appearance of a network. As a stand-alone system, Unix was pretty good. But when you networked Unix machines together, you got a network of stand-alone systems instead of a seamless, integrated networked system. Instead of one big file system, one user community, one secure setup uniting your network of machines, you had a hodgepodge of workarounds to Unix's fundamental design decision that each machine is self-sufficient.
I started keeping a list of these annoyances but it got too long and depressing so I just learned to live with them again. We really are using a 1970s era operating system well past its sell-by date. We get a lot done, and we have fun, but let's face it, the fundamental design of Unix is older than many of the readers of Slashdot, while lots of different, great ideas about computing and networks have been developed in the last 30 years. Using Unix is the computing equivalent of listening only to music by David Cassidy.
Object-oriented design is the roman numerals of computing.