Mass managing collections is currently quite inefficient through the MB UI. Some ideas and things I'm missing are:

• Batch adding/removing items from entity pages, search results and tag pages (loading each page and then having to reload afterwards is a PITA: MBS-10745)
• Moving items from one collection to another en masse once moving is possible (MBS-1806)
• Subscribing/editing/removing multiple collections at once from user collection view (related: MBS-5937)
• Copying items from one collection to another as opposed to moving (see MBS-11465 for one use case)
• Adding items with a list of URLs/MBIDs (e.g. the Darkened collection through the API which didn't seem to work with such a long list? Had to split in multiple parts)