Just in general, because I have no MIDI hardware, easy to set up and configure audio inputs and outputs are a must.
I use Windows 7 mainly. I have a virtual midi synth set up as an audio out (CoolSoft VirtualMIDISynth). It allows me to play back midis using the player of my choice, and any .sf2 soundfont I choose. It also acts as a midi device in (most) older games I play that have midi soundtracks.
Equalizers and sound enhancement options are good to have. Windows varies greatly with this functionality depending on the sound chip present and the version of Windows. Sometimes all you get is stereo with an option to select if you have full range speakers. Sometimes you get a huge array of options for different surround setups, number of speakers, eq, reverb, pitch adjustment, and others.
An audio optimized OS would have all of this functionality by default, without needing to add extra drivers/software.
I use Windows 7 mainly. I have a virtual midi synth set up as an audio out (CoolSoft VirtualMIDISynth). [...] An audio optimized OS would have all of this functionality by default, without needing to add extra drivers/software.
In addition to your comment, I would like to add that VirtualMIDISynth is a replacement for a Windows standard component, the infamous Microsoft GS Software Wavetable Synthesizer, that does the same as VMS but without configurable settings for assigning different soundfonts or change the audio buffer size to reduce latency. I am the author of the VMPK program, and I recommend my Windows users to install VMS for a better user experience.
You say that you don't own any MIDI hardware, but many people has a MIDI controller keyboard or want to buy an inexpensive one with USB connection. If you attach your MIDI controller with an USB cable, install the drivers (if it is not a class compliant device) and want to start playing, using the computer's soft synth. What happens? that you can't. Neither VirtualMIDISynth nor the Microsoft Soft Synth allow to connect a hardware MIDI controller. Why? because the Operating System does not have any MIDI routing functionality. And for a developer, the architecture of VMS is not very affordable either: it includes a system MIDI driver in user space to emulate a hardware MIDI port, because the operating system does not have virtual MIDI ports. MIDI routing and virtual MIDI ports are included in Linux ALSA Sequencer, and also macOS/iOS CoreMIDI. By the way, some people use my VMPK program as a workaround for this Windows deficiency: you connect the MIDI IN of VMPK to the hardware MIDI controller, the MIDI OUT of VMPK to VMS, and enable the MIDI THRU function to pass the MIDI events from the IN to the OUT poort. I am happy that my program could be helpful in this use case, although it was not the planned scenario.
Another use case where Windows is not optimal is if you want to record the MIDI events produced by VMPK or another program in a MIDI DAW program like Cakewalk, or MuseScore. You cannot connect both programs in Windows without installing a Virtual MIDI cable like loopMIDI by Tobias Erichsen (which is, again, an operating system driver that creates two connected virtual MIDI ports) that should be provided by the operating system in an ideal world.