Melian's Change
 

After Thingol died, why was Melian changed. Why were her powers diminished?
Was it the Power of Love? Was it Morgoth's power that had become greater? Was it a design of Manwë?
Why did she abandon her people?

I don't think she lost her powers, she just pulled them out of Middle Earth. ME had become a home for her and Thingol. With Thingol dead, she had no more interest in ME--indeed, no more interest in anything at all. The power she'd infused into Doriath she took back and then she went to Aman.

Didn't she cared about her people? I think that she should have done more to help them.

She did. She handed them over to the governance of her daughter and daughter's husband.

She was broken, in a way, by Thingol's death, and I think it was probably for the better. I don't think she would have been much of a leader.

I believe a union of this nature, of high elf and maiar, is something more than just a marriage. I believe Thingol and Melian were connected at the spiritual level, not just the physical. This would mean when Thingol was slain, that connection was broken and the power was recoiled. Think of it like this: Thingol and Melian's union was a chain that bound all of Doriath. Once half of this chain was broken, then the rest of chain would fail, and the protection fall with it.

agreed, orofacion.

i don't have my silm with me but also from what i understand contextually of their union, melian through such a spiritual bond had invested into or at the least shared with thingol some of her power and wisdom. by this bond thingol the elf was able to rule the great elf-kingdom of doriath; melian the maia shared that rule and with her native powers protected doriath with her girdle. when thingol died i understand she had to withdraw both rule and power because the former was not principally hers and this invalidated the application of the latter. the ainu are destined to rule over the kingdom of arda, not the kingdom of elves. doriath did not begin its ruin at melian's withrawal but rather at the death of thingol, the doom of its ruler.

admittedly this dwells a trifle too much on technicality, but that's how i understood it from the very beginning.

Perhaps this act was of the same nature as with Luthien and Arwen. With their men dead they both chose to die too. Melian being a maia perhaps couldn't die, so she just diminished (?) and left.