ORIGINAL TEXT:
Blessing of the Guilds

Enchantment
For each particular creature type, if no other creature type is shared by more in-play creatures than that particular creature type is, then all creatures with that particular creature type get +1/+1.
REVISED TEXT:
Blessing of the Guilds

Enchantment
Each creature gets +1/+1 for each creature type it has that is the most common creature type among all creatures or tied for the most common.***
One intent of this card is to grant creatures with multiple types more than +1/+1 under some circumstances.
Another intent of this card is to grant +1/+1 to more than one creature type under some circumstances.
Not sure whether there is a better way to word this, or if the phrase "in-play" is needed.
This should results in some interesting situations. For example, with this card in play and nothing else but lands, you play a human wizard. Your human wizard gets +2/+2 (for human and for wizard). Then, your opponent plays an elf wizard. Now, your human wizard only gets +1/+1 (for wizard), and your opponent's elf wizard get +1/+1 (for wizard). On a later turn, with the same creatures in play, your opponent plays two elves (no other types). Now, your human wizard gets no bonus, and each of your opponent's creatures gets +1/+1 (for elf).