Sounds like guild leves will be soloable. So I hope quest lines do require parties since there will already be a solo route.While I actually enjoy leveling in groups, I actually worry about the reverse. I find it frustrating when I hit a level where I'm eligible for a new skill and then have to find a group to help me go out and kill an NM because it's unsoloable for my level, yet they receive really no benefit for helping me other than a pat on the back and a "thanks for helping out".
While WoW had a lot of quests, these type of group quests you would pick up would still be an issue. In most instances they are completely unsoloable for the level they are acquired and when you're usually off doing questing on your own, it's rare that someone else is looking for that same specific quest you are (especially given the wide level range in WoW). I worry more about that here, because it seems that the questing system is going to be very story driven where you'll probably need to follow them in sequence. Usually in WoW I would just skip those group quests and do the other ones that were offered. But if I'm in a questing sequence, now my progression is blocked unless I can find someone else (or a group) on that specific quest, which is very unlikely, or I need to beg/ask someone to take time out of their schedule to come help me with it so I can progress.
I just hope the development team is thinking about this as they create this new questing system. I do love partying and would love to have some quest lines designed for groups around a specific level range. Where you complete a series of 3-5 quests over some period of time (chains of varying lenghts, 1-3 hours). I would like a system like that since all final fantasy's are about the journey of a group. But if the quest chains are mixing solo quests with group quests, I just know I'm going to find myself frustrated having to look for help every time I want to continue the story I started.