1. Difference. Of course, otherwise why should I create it?
2. Advantage. Users should decide, install it and compare.
The co-called Czech-qwerty included with the system, and similar to its Windows or Linux counterparts, simply replaces Z with Y, and vice-versa. A really qwerty keyboard for Czech should include many, many other difference in order to be practical. To add that my keylayout included virtually all the chars required for writing any European language based on the Latin alphabet, e.g. Lithuanian, Latvian, Polish, Maltese, Romanian— and, yes, even Rromany.

I suggest, that you change the info about what your submission in fact does. The name is confusing, because QWERTZ and QWERTY should have just the one difference, which is swapped Z and Y. I don't get get, why Czech keyboard layout should include Lithuanian, Latvian, Polish, Maltese, Romanian etc characters? Call it mutli keyboard then, no?

Not at all, perhaps it would be better to test it and, if you like, use it. I have the feeling you are writing about something you have not tested.
All keyboard layouts in Mac OS allow additional chars not strictly specific to the language in view. Mine includes virtually all, this is no harm to anyone, who does not need them, but most useful to those who do.

That's true, I have not tested it. But I am using Czech QWERTZ keyboard daily and I am trying to find out why it is called Czech kbd layout and what is the purpose. Does it do the same as the new feature introduced in Lion (hold key for other characters)?

Kichi, you are a tireless questioner, it would have been a lot more relevant if you had installed that humnble keylayout and tested it. If you currently use a Czech qwertz keylayout, and you are satisfied with it, this keylayout is NOT for you, very probably. Bohemica Carolina qwerty is NOT targeted at Czech users living in the Czech Rep., but at foreigner studying Czech, and who live abroad, also being accustomed to a qwerty (U.S., U.K. etc.) physical keyboard.