Not being able to progress in DLCs or run DLC dungeons, alter housing furnishings above the non-subscriber limit, deposit things in your bank over the non-subscriber limit, or dye costumes.
Not that I know. You can still craft with items in your bag and withdraw from it. If you spent a lot of time farming before, you're probably good for a while.
One thing that could be a problem is you'll lose the double item limit on your house and double bank space. If you're above the non-subscription limit you'll have to get below the limit before you can add anything. Say for example you have 200 bank spaces - that will go up to 400 with a subscription and if when it ends you have 250 items in your bank you'll have to get rid of 51 before you can deposit anything else. But if you know you're only going to have a subscription temporarily that shouldn't be a problem because you can just make sure you keep your bank 1/2 empty (or fill it with stuff you can easily clear out later).
You also won't be able to dye costumes any more, and like VaranisArano said you'll lose access to any DLC areas you don't own.
PC EU player | She/her/hers | PAWS (Positively Against Wrip-off Stuff) - Say No to Crown Crates!
"Remember in this game we call life that no one said it's fair"
Also keep in mind if the subscription lapses, you will pull out items from the craft bag before your own inventory when crafting (bags, bank). So all new craft mats you pick up will stay where they are in your inventory till the craft bag is depleted of that item. So no keeping a stock in your craft bag while using up your bag items.