You keep all 5 toons, just can't make any more (so make as many toons as you can now before you unsub). You decide which crew skill to put on hold until you resub (you don't lose the levels). You keep your bank tabs, inv slots and your old gear (event and purple) is still equip-able. Your extra credits get put in an escrow account, on hold for when you resub (best advice would be to split your credits across your toons). You keep HK, but cannot access section X without buying a cartel item to unlock it. You keep legacy perks.
I'm not sure about the commendations, but I believe that preferred have the same total cap, only earn less per week. So you wouldn't lose any, just earn less in the future.
If you have a ton of credits, buy some unlocks off the GTN, like Artifact unlock, Section X, Hide Helm