I think many people prefer to attack Assange's character or Wikileaks' goals over those of their own countries because that's the only thing they can picture in the equation.
I would tend to agree and disagree. While some no doubt feel that Assange is against their country and against them, I do think that as a central impact being damage to nations which have done a fair amount of good over the years it's unfair to say many people are doing it for that reason alone. It's not as if America hasn't had it's actions dissected to hell over the last few years. I am willing to bet if I look through GameBanshee, I'll find a few hundred threads on the US, and what it's doing.
Keeping in mind that those not of that opinion are typically getting their information via the media, and not actually from the source. Many of those on forums who have read it are shocked by the general banality of the actual documents, whereas the media are busy touting it as a great release of information.
We're already dealing with a slightly slanted view via the media, and from a man who appears to have a less-than-altruistic goal in mind. It's important to remember the slant of the source before we take it into account. If there is a slant here, than it's incredibly important since it could mean we are not getting the full picture.
Other people are likely opposed because Assange is removing freedoms to privacy. Let's keep in mind also that some diplomacy only occurs because diplomats don't lose face doing it behind the scenes. Applauding the opening of diplomatic ties removes familiarity between parties and the freedoms some privacy allows. If we can get a military dictatorship to agree for concessions in quiet, it does a lot more for everyone than failing to have them agree for concessions in public. Other things are easier to do if they are held quiet, for example, moving troops into areas for interaction with the locals.