For me with mid-fi - you get clarity in two dimensions and with hi-fi you add the third dimension.
Let me explain -
Mid-fi:
in mid-fi you can really get a good enjoyable sound, in fact if you have any sense, you will not want to ever hear a hi-fi system and get bit by that bug! Mid-fi is good - you can tap your foot to it. You can get a chill up and down your spine with the right song. A trumpet sounds like a trumpet, etc. It sounds MUCH better than your boom box, etc. Bass will be pretty good. If you have this system, enjoy! It's a good system.
Hi-fi:
Want to hear the difference between hi-fi and mid-fi fast? Listen to some expert cymbal work, especially jazz, with brushes, etc. You will notice a lot of "space" around that cymbal. It seems to float in the air, and you almost feel like you can
see the brush touch the cymbal because the illusion of it being there is so great. In a nut shell you get more of the third dimension, that sound stage is more real. A great recording on this system not only sends the chill up your spine, but it makes you want to quit your job and remain in that chair 24-7 with your breath being taken away.

You will only come back to reality when the power company cuts off your power for forgetting to pay the bill. The same thing happens with vocals, instruments, bass, etc. But those cymbals really make it easy to hear the difference, I think. With classical, you will be arguing with your buddies on whether or not you are in the third row or the fourth row of the performance. With hi-fi you get more of the "you are there" feeling. Also you will feel your money walking out of your wallet, but try not think about that while listening. :yes:
Hot tip --- if you have never listened to a hi-fi system, don't ever listen to one. Just enjoy your decent mid-fi system and enjoy going out to a live performance. Remember I tried to warn you... Once you get used to hi-fi, the bar is forever raised and the bug will have bit! It is a world of beauty, but it is highly addictive.