Here is how (I think) it will play out
(note, this is based on current information and translations, and I'm just trying to simplify what I got out of it)
You start out as an archer lvl 1 at P.lvl 1
You play dead straight for 8 hours and manage to hit say, Archer and P.lvl 14 (this is making physical level and class level dead even in exp, which is not the case, but bear with me here)
Your exp eared starts to decline for both, and since the exp per level goes up, you manage to hit Archer and P. lvl 20 before you hit the fatigue limit at 15 hours. You can now no longer earn any exp for archer, or any additional P.lvls.
However, you can switch to any other class, and play it to its fatigue limit. Note that since your physical level is 20 for every class, leveling new classes will be much easier. If we assume that each class has the same exp per level (a safe assumption) you can without doing any math know that you can take each class to 20 before hitting fatigue for that class.
Now for a bit of personal opinion-
The overall system seems to be developed with 2 ideals in mind:
1)To stop you from zerging max level with a fresh character, preferring growth over time.
2)To encourage you to pick up a craft, an alternate class, or several of both.
(there are 18 classes, at 15 hours a piece that is 270 hours, or 102 (over 4 days) more hours than there are in an entire week.
I believe that this system effectively combats RMT and powerlevelers (RMT get banned ? no matter how fast they can kill, they can only go so far each week, and hopefully are found and banned again before they reach a level that can have a significant effect on the farming and practices of the "end" game)
I also believe that this will make bandwagoning jobs harder. A job get a cool buff ? While that job has its moment in the spotlight, tards won't be 2 day SMN burning up that job in **** poor gear. Sure if it stays powerful, after weeks a few might pop up, but the game can change again in other jobs favor in that amount of time.
For myself, I'll be playing archer. There are several job skills that work nicely with archer, and assuming they get more powerful versions of those skills later on, I'll be leveling the jobs that have those skills with my archer, to make my archer more powerful. Already I'm looking at both other DoW for more raw power and a magic class for bind (on top of shadow bind) for more soloing prowess. I have no doubt in my mind a melee class will get some sort of Req. DoW berserk, and everyone that hits things will want it, and the people who leveled that class along their other damage dealer will rejoice.
In addition, Leveling just one DoW, (tank or melee) one DoH/L(craft) and one DoM(magic) to full each week takes 45 hours. That is over the amount over time for a full time salary at a job in the real world. It comes back to not forgetting friends, family, or work/responsibilities. If you worked full time and did that, and got say, 6 hours of sleep a night, that is 125 of your 168 hours a week taken up. That doesn't count eating, bathing, sexing your gamer girlfriend- nothing. And lastly, people act as if once you hit the limit ffxiv will just outright shutdown and you can't log on for a week. You can do guildleves for gold, you can kill animals for materials that will make gear that will improve your character just like it kept leveling. ****, you can level something off the wall just to see if it works (does gladiators still precision +acc -eva effect my ranged attacks ? only one way to find out !) Although this is mostly for beta and players getting the game right off the bat, as I'm sure people will be wiki-ing their findings and will actually save future adventures a lot of trouble.
As for me, this has lead to a firm decision of buying the standard edition. My name is fairly unique so I don't have to worry about that, however, I now know exactly how far ahead of me the pre-orders are in terms of my favorite job versus theirs - 15 hours.
TLDNR : Buying it, and enjoying my relaxed and fun (like a, game should be) time to level capsville.