The obvious things are:
- ending it voluntarily
- losing concentration
- breaking the tether with range or cover
- using a normal action on your turn for something other than witch bolt
What isn't specified is whether or not the spell ends when you use a bonus action, movement or reaction. I would assume that reactions would be fine because they are quick spells and not on your turn. Movement is generally not considered an action in 5e afaik, so it seems fine too. The description is less clear about bonus actions however, because they occur on your turn and witch bolt states
On a hit, the target takes 1d12 lightning damage, and on each of your turns for the duration, you can use your action to deal 1d12 lightning damage to the target automatically. The spell ends if you use your action to do anything else. The spell also ends if the target is ever outside the spell’s range or if it has total cover from you.
BUT Bonus actions are fast and often without a somatic/verbal component, and most cases in the PHB refer to the normal action as "your action". Also, the action required for witch bolt is NOT casting a spell and so the "bonus spell + cantrip" rule is not applicable here. Witch bolt is presumably taking a mental effort to maintain, such that I wouldn't be able to cast another normal action spell without losing focus on the beam, but for something like misty step, with only a verbal component and taking minimal effort (especially if it's a wizard spell-mastery spell), it seems to me like it would be possible.
Anyone know if there's an official ruling for this, or is it entirely up to the DM? If I were a DM I'd rule that it's allowed, since it requires giving up your standard action, and you're still limited to the 30ft tether AND line of sight, AND concentration, AND spell slots if you're not a lvl 18 wizard. Compared to a high level blastlock hitting for 4d10+20+knockback with eblast each turn, with a cantrip that would allow a bonus spell on your turn and a reaction spell on someone else's turn, witch bolt with bonus action and reaction (now that I think about the math) still seems underpowered at high levels. (especially now that I realize that subsequent turns don't add the extra damage from a higher lvl spell slot. A lot of text for something not terrible useful. Still curious though.
TL:DR Can I maintain witch bolt on a target and move normally and use my bonus action to teleport around with misty step (provided I maintain line of sight and tether range), while using my reactions on enemy creature's turns for counterspell or shield?
You said it yourself, witch bolt "ends if you use your action to do anything else". Using your bonus action isn't using your action.
In 5e, it's just "action" not "standard" action, and as you also said "movement" actions aren't a thing. PH 189 says that "On your turn, you can move a distance up to your speed and take one action."
If you spend that one action doing not-witch bolt, the spell ends. Anything else you do with your move, bonus action, or reaction are immaterial—unless those actions violate one of the restrictions of the spell you already listed (like using your bonus action to misty step out of line of sight).