Hmm... last time I checked, Catholic priests were not reknowned for their martial abilities... hehe. And the sterotypical weapon of the "friars" is the staff... which Monks cannot use... but then again the D&D world can be an odd place...
Well, obviously that's the main thing taken from Shaolin monks
. The "Catholic aspect" is more the way they are devoted purely to their god and his/her commandments, rather than in pursuit of inner peace and understanding as are Shaolin monks. The most obvious consequence of this is that D&D monks generally pray, rather than chant or meditate.
BTW becoming a Catholic monk doesn't automatically make you a member of the clergy; in fact, relatively few of them are allowed to be ordained as priests
. Catholic monks and friars weren't quite the same thing either; whereas monks usually shut themselves away from the rest of the society (either by themselves or in a monastic order) and devoted themselves purely to God and the pursuit of His teachings, a friar lived amongst a normal community and was dedicated to helping the ordinary people in matters both spiritual and otherwise (and he had both fewer and less strict vows as a consequence).