To address your first question: Steam has a few identification means, first being your username which is used specifically to log into your account. This is unique to you and no one else can have the same user name, and to the best of my knowledge only you can see it (it's not listed anywhere else unless you list it elsewhere). Second is your steam community URL. Again this is unique to you and once you select it you can't change it. The URL is associated with your profile for use in the community through the Steam client or web browser. Essentially your URL is a public name linked solely to you, while a username is entirely private. Lastly is your display name, this name can be just about anything you want, even if someone else already has it. It's the name that will appear in-game, and it appears on posts and on your profile throughout the community.
So for example:
The above link is a search within the Steam community for all users with the name Josh publicly assassinated with their account. Key word being "publicly" If you real name is Josh and you have your real name set to private, your real name will not be included in the search criteria if someone were to search for you.
The other area where it differs from XBL and PSN is that Steam ID only works within Steam. and Steam is not the only digital distribution client on the market, It is certainly the largest though that sell games not just from Valve, but over 6,000 games from 100s of other developer companies.
EA for example has their own client in which they sell just about all games published by EA, They don't sell non-EA games though.
Blizzard is similar with Battle.net
And of course there are non-client based digital distributors.
For your questions on quality and noticing a difference:
Personally I only notice a difference when I go down in quality of performance. Going up in quality can be noticed but it doesn't feel impactful. Where as going down, you'll immediately pick up on what you've lost. Defiantly so with frame rate; going up from 30 fps to 60 fps you won't notice a difference, but once you get use to 60 fps and you go back down to 30 it will feel like the game is unplayable choppy until you once again get use to 30 fps.