TL;DR
It would be nice if one could set the filter for loaders to blacklist items instead of whitelist.What ?
Filter inserters have the option to choose between a whitelist or blacklist of items. The same option should exist for loaders,Why ?
Playing with angles mod there are many repeating setups that only differ in the angles ore they deal with. For example all 6 angels ores need to be crushed resulting in the crushed ore and rushed stone as side product. Those then need to be separated. Using filter inserters one can use two of them, one set to whitelist crushed stone and one set to blacklist crushed stone. This makes it simple to blueprint the ore crushers for different ores.Not so when using loaders. Now each loader has to be programmed for the specific crushed ore.
Note: For splitters a blacklist for the right side is simply a whitelist for the left side. So they kind of have this option too. Only loaders don't.