The ending of Quantum of Solace closes Bond's revenge story: Greene dies in the desert, Camille has her revenge on Medrano, and Bond finally puts Vesper's betrayal behind him. After the hotel fight, Bond leaves Greene stranded with only a can of oil, and M later learns Greene has been found dead. Bond did not pull the trigger, and that matters, because the film spends its 106 minutes asking whether he can stop treating every kill as payback.
The structure of the final act supports that reading. The fuel cell hotel in the Bolivian desert is where Greene and Medrano complete their land-for-power deal, and it is also where the two revenge plots meet. Camille settles her account with Medrano, who murdered her family, while Bond's target is a businessman whose crime is control of Bolivia's water supply. Bond's choice to strand Greene rather than shoot him is the clearest sign of the change M feared he could not make. She cancelled his passports and cards in the middle of the film because she suspected vengeance was driving him. The desert is where that suspicion is tested, and Bond leaves the killing to the desert and to Greene's own associates.
The quiet final scene carries the real weight. He drops Vesper's necklace in the snow and walks away, which marks the end of the Casino Royale story rather than a throwaway beat. Quantum of Solace is the 22nd film and Casino Royale the 21st, and they are the only two consecutive entries in the official EON run of 25 films that resolve a single emotional arc between them. Paired with his line to M that he never left the service, the necklace answers her worry from the first act: his anger never cost him his loyalty. Mr. White's capture at the start and the traitor Bond hands to MI6 near the end bracket the film, so the personal thread is closed where it opened. The film has no post credit scene, so the story stays on that image.
The secret group is still active when the film stops. That is why Quantum of Solace reads as a bridge more than a finish, since the personal thread is tied off while the larger one is left open. Skyfall, the 23rd film, is the next entry in release order. The title says the same thing: it comes from a 1960 Ian Fleming short story, as Screen Daily reported when it was announced on January 24, 2008, and a quantum of solace is a small measure of comfort, which is all Bond gets.
Production context points the same way. Sony and MGM moved the US release to November 14, 2008, so the film could play through Thanksgiving and into Christmas, and the finale was shot in Chile around Baquedano and the Atacama area standing in for Bolivia. In my opinion, the sparse desert setting suits an ending about restraint, since there is no lair to destroy, only an empty landscape and a can of oil.
For where this film lands among the others, the James Bond movies ranked page sets it against every official EON film.