I'm getting ready to move this into the 'general discussion' forum; that said, I'm going to chime in as well. Korean/Chinese resentment towards Japan was actually not all that large during the course of the Cold War - WWII in a lot of ways was pretty much moved on from. In the late 90s though there was a resurgence in resentment from both countries as a result of increased nationalism at home and increasing resource squabbles with Japan, not to mention ever merging/conflicting economic interests. Sometimes I think even the Japanese themselves are unaware of the shifts in perception in recent times (they tend to insulate themselves from foreign criticism), peaking with what really became an active propaganda campaign of sorts by the Chinese government to highlight Japanese atrocities during WWII as a public awareness/support move to check what was/is perceived as growing Japanese willingness to re-militarize (which is indeed happening). So... since the natural rising 'power' in Asia is China, and Japan of course could go nuclear in a year if they wanted, you get all the Nanjing talk, the comfort women stuff, etc etc... all to highlight the potential threat of Japan.
Now - things that have *highly* aggravated the situation in recent years have been Japan's willingness to re-work their history books to play down/pretty-up their actions during WWII (at China and Korea's expense), former Prime Minister Kozuimi's frequent visits to Yasukuni Shrine (where several class A war criminals are still honored), clashes over natural gas rights in the seas, and a move to re-write the constitution in order to allow proactive military action.
In fact, the success enjoyed in raising awareness for these issues actually started to backfire for the Chinese government, as people actually did start boycotting Japanese goods, and Japanese businesses and employees working in China started to feel a little less secure. China's goals were only ever political; they never wanted to endanger the economic relationship, so the government-generated rhetoric has died down in recent years to let things cool off again. A new PM in Japan doesn't hurt things from that standpoint.
That's a quick overview of the situation; it was better in the 80's than it was this past decade, and it's better now than it was just a couple of year ago.