MMORPG.com has interviewed James Ohlen and Gabe Amantangelo and posted it online. There is much talk of PvP, interesting if you are into that sort of thing, and some nuggets about replayability via alting. Sith and Republic characters have totally different quests, but reroll on the same faction and things will be familiar:

However if you decide to play a Bounty Hunter and then play a Sith Warrior, your origin worlds will be completely different, and then once you get to the capital world the content sort of “blends” together. Your class content is going to be completely different, but the smaller side quests will be shared by all.

I guess this will be ok or sucky depending on how much the class quest actually takes up of your total questing. They go on to say you can skip zones or quests, so you can minimize repeating content. But this won't work for your 5th alt.

No one has pressed Bioware about grouping up with two different classes either. There was some talk that you would have to travel to a different planet than your starter one to group with a friend who has a different class, in the lower levels. But maybe at higher levels our class quests all take place on the same planets, and so we can group up, doing one person's quests then the others. I guess in this respect, having non-class quests actually helps, as everyone in the same zone is usually on the same set of quests, and grouping becomes easy.