The two big perks for me:

1. Exit Row. I am taller than average and having the extra leg room makes a huge difference.

2. Fewer fees. I can make Award bookings last minute, or change existing Awards (even cancel) without any fees. This alone saves me over $1000/year.

I'm sure lounge access for international flights, higher priority for Standy-By, Same-Day Stand-By without a fee, and Priority Access for Security lines and early boarding (to get overhead space) make it appealing to others.

It's not all about First Class (though I've never turned it down).