Back in the day, to get your new medallion status to show up on previously purchased itins (so you can get ug's, preferred seats and such) you need to call and have PSS 're-insert' your SM # into the res. This opens up the preferred seating and puts you on the ug list. I'm not sure if this is still the case as it's been years since I had to go through that process. It's not painful, just a quick phonecall.

You can already see the preferred seats, you just can't select them. Other than exit rows, they're no different than any other coach seat. You will get to board in zone 2, before the unwashed masses that you have just emerged from. It's a big deal as you are now all but guaranteed overhead space right at your seat.

As for upgrades, well, unless you can choose your travel and select times when no one normal wants to travel, don't count on one. In fact, hold very little hope and rejoice when you do get one. That's not to say it won't happen. I was 90% last year as FO (silver). As I travel for (mostly) pleasure, I carefully select my flights so I have the best chances.

Other than that, not much different. Shorter wait times on the phone, slightly better treatment during irrops, 25% bonus on SM accrual, basically stuff from the list Crackerpup links to.