Like c47, I too am waiting for a real reason why the caster cannot ride on the disk. I have always seen the caster being able to ride the disk.
Generally, a spell (or use of a spell) is considered "Unbalanced" if it:
A) Can be used to emulate the effects of a higher level spell (alone, or in conjunction with a spell of equal or lower level).
B) Has a property that makes it unstoppable and/or impossible to counter (alone, or combined with a spell of equal or lower level).
C) Deals damage/has ill effects that is not on par with other spells of it's level
D) Provides the user with the ability to circumvent obsticles inappropriate for the spell level.
Note: Use the PHB as a baseline.
This is of course not an official list, just my $0.02. I would love to see additions/revisions. I left out "Because the Sage said so, and because the errata said so" tests because I feel there should be reason behind a ruling. When spell combinations come into play, it is important to analyze which spell is unbalanced.
So, in regards to the above list:
A) Floating disk most closely emulates the Mount spell when riding on it (Because the disk must be 3 ft from the ground). The disk has a much shorter duration, but can carry more weight, and for the most part will move slower than the mount spell, even when using Expeditious Retreat, so I see no problems there.
B & C) The disk does not deal damage, nor when riding it can the caster gain and combat related advantages that cannot be gained from riding a horse. so riding the disk does not apply.
D) This category is always tricky. It has already been brought up that the caster could ride on the disk over water, which could be considered an abusive use of the spell. Thats DM discretion. I see it as only slightly better than a rowboat, and emulating the awe-inspiring power of a wooden rowboat is not performed by any other spell in the PHB.
So unless someone can come up with a good argument against riding on the disk, im all for it.
