The aggregator module in 4.5 and 4.5.1 hardcodes the default "Filtered HTML" filter, as opposed to hooking into the filter.module system itself. This can cause problems for a number of feeds (see http://drupal.org/node/13283 for an example), since [IMG] is not one of the allowed tags. It also greatly causes issues for users of Flickr (see http://flickr.com/forums/help/2943/ for a Drupal specific issue). If aggregator.module could hook into the default filter.module, users would be able to create a "Aggregator" filter which includes tags they'd like to see (like [IMG]). The first stage of the feature should be "support a single filter for all aggregated items" and the second stage should be "support a specific filter for a specific feed, with a default for all unspecified". The default, out of the box, behavior, should be to use the "Filtered HTML" filter.