- the entire desktop metaphor (tired, outdated, can be improved upon)
- the entire hierarchical filesystem metaphor likewise
- application installation
- the registry
- WinSxs - seriously, take a look at how much space this directory "wastes" (increasingly important with virtualisation and SSDs)
- the start menu - it's not useable, honestly
- ribbon UI (don't even get me started)
- the whole look and feel of Windows (more akin to Fisher Price than polished glass and aluminium)
- the quality of bundled applications - notepad and paint - seriously? in 2011?
- I could go on and on and on. If it weren't for the quality of the .NET stack and the jobs market for .NET I would be on Linux/OSX in a flash
IMO, Windows merely reflects the beuraucratic uninspired organisation that creates it. This will never change, so really, I shouldn't get so upset, but I have to work with this goddam OS, so it grates a little.