Is there any CR limit of what you can turn yourself into? My idea is to transform into a high ranking demon that can fly.

"The new form can be any beast whose challenge rating is equal to or less than the target's (or the target's level, if it doesn't have a challenge rating)."

Vallid polymorph-into options must be:

• Beasts
• CR ≤ target's CR or Level

So no polying into a demon.

Quote from Combatkirby >>

Can I use it on a object?

is an object a creature? Or is an object an object?

Cmon. 

Let's bring this all together.

• The target of the spell can be any type of creature, but it must be a creature. As noted above, objects aren't creatures (until you use animate objects to turn them into creatures... but that's another story).
• The shape the creature takes must be that of a beast. Not an aberration, or a fiend, an undead, or a monstrosity. Beast, and only beast. The form must also have a CR less than or equal to the target's CR or level.
• All of the target's game stats are replaced by that of the new form. ALL of the stats. This includes ability scores, race features, class features... EVERYTHING.
• If the creature reaches zero hit points or dies in its polymorphed state, it changes back into its own form with all the hit points it had before changing form, less any excess damage if it is reverting due to being reduced to zero hit points. If it dies without being reduced to zero hit points (e.g. power word kill), there is no excess damage to transfer.

The spell description answers a lot of the questions that have been raised.

Would a monk polymorphed into a giant ape still have monk abilities?
No. All of the monk's game stats are replaced by those of a giant ape.

Could a caster maintain concentration after casting the spell on themselves?
Possibly. Since the spell was already cast, losing the spellcasting ability wouldn't end the spell. Detect thoughts implies that a creature with Int 3 or less has no thoughts. If the beast form has Int 3 or less, it's likely it wouldn't be able to keep concentration. Higher Int, however, could easily still be capable of concentrating on the spell, using the Con of its new form to make saves if it takes damage.

Can power word kill be used to kill a polymorphed creature?
Yes, but it only kills the polymorphed form. Here's the order of operations.

1. Creature is polymorphed into, say, a 1 hit point mouse.
2. Polymorphed creature is subjected to power word kill.
3. Polymorphed creature dies.
4. Polymorphed creature reverts to its original form.
5. When the creature reverts to its new form, it returns to the number of hit points it had before it is transformed. Assuming this is non-zero, it is alive again.

The general rule that dead creatures can't regain hit points prevents effects such as cure wounds from restoring the dead by adding hit points. In the case of polymorph, specific rules override general. The dead creature isn't truly regaining hit points. It's returning to its alive, pre-polymorphed state.

Can a fiend warlock eventually cast polymorph?

Quote from All_Seeing_Mage >>

Can a fiend warlock eventually cast polymorph?

 If you take the Sculptor of Flesh invocation you can cast it once per long rest using a spell slot. Otherwise, it's not on the Warlock spell list.

"The spell has no effect on a shapechanger" - So... does this work on Changelings? How about Druids? Plasmoids?

Quote from Wurutana >>

"The spell has no effect on a shapechanger" - So... does this work on Changelings? How about Druids? Plasmoids?

This is not very well specified in the rules, but I think the intent is for creatures who actually have a "Shapechanger" feature in their stat block — such as Imps, Mimics, or Doppelgangers — to be immune to it. That said, I think it would be reasonable to argue that it shouldn't work on a Changeling or a Druid. Plasmoids would be a bit more of a stretch in my opinion.

Quote from NedyBonz >>

Quote from SamsonSlice >>

I imagine you would be restrained at the very least and maybe start taking crushing damage each round.

 This is the same conclusion that I came to, though I have yet to decide the precise rolls involved.

 Maybe 2d12

I have plans for a hag that has a belt with rats on them. occasionally the hag may snap the neck of the rat and throw it you might be wondering what would the point be to that its just a dead rat. it is actually a polymorphed troll so..... TACTICAL TROLL.

Quote from NedyBonz >>

What happens if polymorph's target is in a space that will not accommodate the new form? For instance polymorphing another character into a T-Rex (huge beast) in a cramped 6 ft. by 6ft. tunnel?

 If the creature doesn't fit into the available space,  I would first look at the squeezing rules (chapter 9 - Basic Rules). If the creature is still too large (i.e. 2 size categories or more) then it would take crushing damage for both the creature and the structure equal to the lesser of the HP of the creature or the structure. So, unless you are in a flimsy structure like a wooden shack, the creature's HP would be reduced to 0, reverting it to its original form, and thus wasting the spell.

Squeezing into a Smaller Space

A creature can squeeze through a space that is large enough for a creature one size smaller than it. Thus, a Large creature can squeeze through a passage that's only 5 feet wide. While squeezing through a space, a creature must spend 1 extra foot for every foot it moves there, and it has disadvantage on attack rolls and Dexterity saving throws. Attack rolls against the creature have advantage while it's in the smaller space.

Yess

Can you polymorph a ghost?

My players have several times polymorphed huge/giant opponents into turtles (slow and easy to handle), then stuffed them into cramped spaces and ended the spell, thus chrushing the monster. Alternatively have they transformed the foe into a worm, let a familar fly it high into the sky and returned it to its natural form just after letting go, letting gravity do the work.
As a DM I love the creativity but also hate that most foes easily can be defeated this way and have yet another encounter become ridiculously easy, so I wonder how to limit the use aside from avoiding any cracks and open skies. Of course, I can have one more monster than they can transform, but that can also be a problem to match the story etc.

Another thing: Is the transformation instant or gradually during a round? The PC's wanted to transform an ally in a 15' wide hallway, and we discussed whether that would crush the people next to it or if they would be pushed away instead.

How do I add the creature as a warlock? As in the creature I am morphed into

Technically yes, but it would end immediately becasue your concentration would be broken.

Hey in my dnd group we polymorph a party member and they got shot out of it. Is there orientation still the same from his original form or does it change with the orientation of the creature