iTunes is just shit when it comes to tags.

I use Foobar and I can create whatever custom field I want, and I can set up a search to search by keywords or whatever.

Editing and adding fields: http://i.imgur.com/8Pv3JcR.jpg

You can create and edit fields in various tagging programs (e.g., Picard, mp3tag), but iTunes just won't use or see them. For example, take the "Date" field. iTunes will only allow you to see/use the year, but you can't see or use months and days in the field (I think iTunes purchases do use them, but you're SOL for you're own stuff). This means that when you try to sort chronically by release date, you can only sort by the year but not also month and day. This is problematic (at least for me) when artists put out multiple releases in a year, and iTunes will just sort things alphabetically if they are released in the same year.

I can't comment too much on the social aspect. I prefer my music player to just play music and not be bloated with unnecessary things like social integration or a store, and something like last.fm run separately is enough for me.