Mechanical, PBT keys, and quiet(to a reasonable degree). I don't use any of the extra features that some boards have and I'd rather just keep it simple, and once you've used PBT keys you really can't go back. One option I think you should add is 'stealth' keys. I've got one of those Ducky boards where the letters are engraved into the keys but not filled making the keys look all black in most lighting conditions. It helps make the desk look less cluttered having it this way and not being able to see the keys helps you type. At first I was nervous I wouldn't be able to use a board without keys but I adapted real fast.
Unfortunately my board has Cherry Blues so it doesn't really fit the quiet criteria. The quiet switches tend to be the ones with low activation force or ones without any feedback. I tried using a board with reds once. It was quiet, but I'm way too heavy handed for those switches. I ended up pushing keys when I was just trying to rest my hand over them.