Sounds like we will likely have classes and this I like. I hope they pull off the balance between single/multi gameplay well. LOTRO did it very well but SWTOR not so much.
LOTRO does it fairly well - certainly for main story they seem to be on top of it, after revisiting volumes 1 and 2.
LOTRO does annoy me when it comes to general questing 'cos they actually make it a pain if you do this with others - I have several characters I duo with a friend and we also have a 3rd person along with us for certain characters.
Kill quests work well, collect x amount of generally means alot of waiting around for items to respawn so all 3 of us can get them (few parts of the last expansion do allow one person to collect the items with people in the group receiving credit but only in certain parts of it).
The other major pain is instanced quests, where we're forced to break the group up all too often sadly.
Making the main storyline soloable is always a great idea - some people work odd hours or later on people are levelling alts or there are new players who struggle to find a full group that will help/need it also.
One thing I would implore ZOS to do is make sure general quests objectives, whether it's kill or collect etc, are shared with groups if players choose to group up - I realise this means groups can trivialise content sometimes but for those of us who enjoy playing with friends we shouldn't be penalised.