Grandm hi hi /wave!!
Here is how it goes. You do a quest and get a job stone then equip it and bam!! you have all your class abilities and whatever job abilities you unlock from doing the job quests. What you lose is most of your crossclass abilities (this is still unknown if its all crossclass or just most of them) but you become even more defined in your role with that class/job. You do not lose any class abilities so don't worry about that... one way to look at it is like this. At level 50 you will have 15 abilities total and 10 crossclass abilities but when you set your job you lose those 10 crossclass and pick up your job abilities (which I think right now they show about 5 or so each job) which pretty much replace the crossclass set and make you a more defined tank/healer/dd.
I hope this helps ^^
The cat is bemused =-.-=