Pentagon papers had to do with actual inappropriate actions of the govt , covert and kept away from the congress. Diplomatic cables are not covert BUT are a classified discussions. What did wiki leaks ultimate show? internal discussions and opinions being discussed? what govt transparency / action that the congress was supposed to know was reveled ?
The diplomatic cables are not the only thing released. There are also the videos of the helicopter attacks in Iraq, the Stratfor leaks, and the numerous war logs of the Afghanistan and Iraq conflicts. This is only for America, not including the data on Iran, big banks, toxic dumping and etc. This stuff is relevant. But it is perceived as putting the US a bad light, when the real intent is simply to have free information. He has done work exposing other countries and companies yet he is accused of being anti-American.
Nations such as Russia, China and Australia don't support Assange because of the various leaks that have been released like the Australian censorship list. And you'll be surprised to know that quite a few times, Wikileaks was taken down by Russian and Chinese hackers for its work in exposing the truth.
For the diplomatic cables:
The idea is that Assange is a journalist. Manning is the leak.
I feel sorry for Manning, but he has broken a law by leaking classified military secrets as a member of the United States military. I wish he would get a lenient sentence, but that's unlikely, since the maximum punishment is execution.
Assange on the other hand is the person who received the leaks, like any journalist when receiving a tip. No matter what you think or how distasteful you think the leaks are, there is no reason for Assange to be declared an enemy of the state and hunted down. Journalists are never treated this way, but only in Assange's case. This kind of extrajudicial action wrecks the credibility of the US in my opinion.
The case that the US wishes to charge Assange on is that Assange somehow coerced Manning into leaking the information. There's no doubt that the US wishes to persuade Manning into testifying that Assange did more than just provide a method to receive the leaks and was an active conspirator.