(there's so much speculation in this you could dip a cookie in it, take a bite, and go "Mmm, speculation".)
In FFXI, after a certain level it became a neccesity to form a full party if you wanted to level with any speed. Forming a full and balanced party could take a while (it's taken me a few hours before), and maintaining one could be more difficult than leveling itself. The solo factor has become one of the biggest complaints of FFXI, and they're claiming to have solved this issue in FFXIV.
In FFXI, soloing up at the beginning was fairly easy; you simply walk outside of your starting city and start attacking enemies in that immediate area. When you leveled above those enemies you then expanded your "attack-zone" to include stronger enemies. Simply put, the further you traveled from the city the more difficult the enemies were, the more exp they gave. I believe this is how it'll be set-up, but on a much larger scale.
Every zone will have a focus, a starting area that holds enemies equal to the minimum level required to enter that area. These starting areas will likely be located right in the entrace/exit line located to the closest starting city. The level of difficulty will sphere out in layers, each layer encompassing enemies of a higher degree.
So, if you are soloing, once you've mastered all the monsters up to the entrace of a new area, then it's time to enter that area. This of course won't be the case for all areas, just the main ones connecting your starting city to the main hub of all other nations (a Juno for example). There will be a peppering of side-areas and dungeons that'll be deadly for all but the more experienced adventurers, but you should get the idea you're not ready for that area from the first monsters you spot. For the most part though, traveling solo to the hub should be easily solo-able.
Forming a party will have it's advantages though. By forming a party, not only will you be able to access areas you would never consider solo at your level, but you'll level much quicker. Even though FFXIV lacks a traditional exp system, it still will work very simular to one. Just as how defeating a high level mob will grant lots of exp in FFXI, in FFXIV performing a successful action upon a high level mob should grant great skill points. Sure, because the mob is higher, then success will be lower, but due to the fact you're in a party it should even out. If, for example, soloing grants you an average of 1-3 skill points per battle, being in a party should grant around 2-7 per battle.
I believe this is how the system will be set-up. Soloing will be available for those that don't feel like a party, but forming a party will always be in your best interest. Therefor we should expect the best of both worlds; a healthy support of gamers of all play-styles, and the freindly natured community we've come to love from FFXI.
Edited, Jul 29th 2010 3:11am by Futzbucket