When attempting to a BBOX CQL filter against geometric shapes like POLYGON, GeoServer errors out with an exception:
org.geotools.filter.LiteralExpressionImpl cannot be cast to org.opengis.filter.Filter
According to the documentation at http://docs.geoserver.org/2.7.1/user/filter/ecql_reference.html#spatial-predicate it seems like you should be able to do a BBOX query against a geometric shape like a POLYGON. Other spatial predicates appear to work correctly.
An example of this issue is the following request: