1. The word of the era is "mainstream." Contemporary jazz and rock has definitely evolved, but it's not as noticeable as the evolution in hip-hop/rap and pop, which are not original genres; rather, they're influenced by previous eras.
2. For sure, certain artists will be remembered, but it'll be more for their public persona, like Britney Spears, Beyonce and Justin Timberlake. As far as artists/songs that have "defined" the era with their music, I'm not so sure. I guess it just depends on who will stay in the business for 10-20 more years, as Madonna has shown since the 80s. If I had to make a guess? I'd stay with the really young artists who have the potential to keep growing, which includes Rihanna and Chris Brown (and JoJo, I hope). Also, even though this might sound really naive and way too early to judge, I could see Leona Lewis being around for a long time.
3. Honestly, that's the hardest question of the bunch. Because there's a difference between remembering a song from up to 8 years ago when I was really young, and remembering a song made 20+ years ago. Again, I'd have to make wild guesses. I'll say "Umbrella" by Rihanna, because that's the song that really showcased her staying power in this industry.
4. I think sampling is a great technique when used right, which I'd have to say, is the majority of the time. Notable exception: Beautiful Girls

And I worship throwback albums. It makes a statement that we shouldn't forget the people who really helped music progress.
5. I think Justin's What Goes Around would make a great sample someday in the future.