There is shared content in leveling on the same faction, I mean thats how group content works. But your class story is completely different, from Trooper to Jedi Knight. If you play a Sith Marauder, then Play a Jedi Sage, about ALL the content will be different.
Raiding usually has enough of a barrier to entry that the majority of players never go raiding. I'm thinking with 8 man (or 16 man) raiding, no crazy pre-raid hoops to jump through (they said, once you hit 50, you're ready to go), or needing pre-raid gear (they said you will go into raids with your story gear), the barrier is much lower. Oh, everyone gets a loot bag after a boss. No one goes home empty handed.
Not to mention endgame flashpoints/dungeons (all the older dungeons will have lvl 50 variants, with I presume nice rewards inside), that don't need a full party to be run. Just a friend, and your best companions, though they have said that will be stupid hard. And last but not least, the Planet Ilum. It's the "endgame planet", with content just for max level players. Not sure what goes on there, but at Comic-Con they said there would tons of stuff for solo endgame.