1. Banning a language because the speakers murdered a bunch of your brother's - originally your own - people wasn't an overreaction??? He had to do SOMETHING - a lot of this people were probably realated to the Teleri + the Noldor lied to him + Melian called the sons of Feanor 'Fell and fey' even before the second kinslaying - letting them into Doriath wasn't a great idea when they'd already proven themselves capable of outright killing Teleri (ethnically similar to the Sindar) and abandoning their own fellow Noldor to a journey that killed half of them
2. The Sindar really weren't in any kind of shape to be fighting any wars??? The First Battle killed A LOT of them - remember, Thingol's people only won one front. Cirdan was still being besieged. Melain put up the girdle because they RETREATED to Menegroth. Clearly they weren't in any place to be waging war.
3. The Eldar didn't come to Beleriand to help them?? Thingol had to live with the fact that his people were dying because they made the choice to wait for him: are those really the kind of people whose lives you'd be willing to risk for some jewels and an overblown desire for revenge for the death of ONE man. His brother Elmo was dead; his children nephews and grandnephews too. The Sindar were used to death.
The Noldor weren't there to help the Sindar or Avari. Why should Thingol risk his people for their sake?
4. He wasn't indiscriminately against all Noldor - he was willing to forgive the Nolofinwians for their (unwitting) part in the kinslayings and only asked Finrod and Galadriel to leave TEMPORARYLY. He was okay with Galadriel learning from his wife and even marrying his grandnephew.
5. The Noldor came to Beleriand partly looking for lands to rule. Could you really blame Thingol for not letting them into his kingdom? For not trusting that KINSLAYERS wouldn't unnecessarily sacrifice his people or make them bear the brunt of the fighting to make them easier pickings later?
Can you really blame him for refusing to join the Union of a guy whose brothers kidnapped and tried killing his daughter? Who killed the Teleri for BOATS? Who were willing to let their own people died out of pride and jealousy?
And given how widely known doom of Mandos was, Thingol would have know that any attempt by the Noldor to fight Melkor would end in Doom regardless of their intentions. As such, staying as far away from them as possible made sense.
6. The Luthien thing was bad. Nothing can make up for that.
But I think the fact that he did it out of love and to stop his daughter from making a what he saw as a huge mistake doesn't make him a bad man? He knew men weren't immortal and didn't want his only child to fade away when Beren died.
And the fact that he made Dior his heir, took in human refugees and fostered Turin showed that he learnt from his mistakes and was willing to change.
Also, Elrond set a crazy high bride-price for Arwen too - Aragorn had to become king of Gondor (which, let's remember, he wasn't really heir to and which had turned down his ancestor's far better claim) AND basically raise Arnor from scratch. Turgon let Idril marry Tuor only because he was blessed by freaking Ulmo AND Turgon owed his father and uncle big time (sacrificing themselves and their people to buy Turgon and his people time to escape).
7. The concept of wereglid was established in Tolkien's work. Luthien had every right to the Silmaril SHE recovered. And not just because she recovered it, but because Celegorm and Curufin kidnapped her to try forcing her into marriage???
It's clear that marriage is pretty holy for elves and NOT something they do for political gain. It's not the same thing as a human being forced into a politically motivated marriage, and as such, is a pretty massive crime she was entitled recompense for.