We do express our opinions but we choose a different way according to a relationship and a situation. It’s true we don’t like to disturb the harmony, and negative opinions are harder to express as people find them difficult to accept. So we have to choose a softer and more acceptable way to convey our negative opinions. There are many speech styles in Japanese but politeness and formality do not always match up. Level of formality depends on the distance between the speaker and the hearer and on any level there are both polite ways and rude ways (or offensive ways). If you want to express your opinions without offending the others, always look for more polite ways. Politeness is about how much you care about others. For example, when you go shopping with your close friend and she asks you if a dress she has chosen looks good on her. If you think it does, you would say, 　「うん、にあうよ」 ”Yeah, it looks good on you!” What if you think it doesn’t? If you are really good friends you could honestly say, 　「にあわないと おもう」 ”I don’t think it looks good on you.” If you don’t want to offend her in any way, you should seek for more polite ways. But never just change the formality. 　「にあわないと おもいます」 That would offend her. You should change the level of politeness without changing the level of formality. 　「う～ん、どうかなあ？」 ”I’m not sure...” 　「ちょっと イマイチかも…」 ”Maybe not the greatest...” 　「そのいろより、あっちの いろのほうが にあうと おもうよ」 　“I think you would look much better with that color than this color” We use various modality expressions according to how we want to present “a fact” or “our opinions”. Non verbal expressions can also convey our opinions. Proper use of sentence-final particles and intonations really matters. It is also true that we often take a “better leave it unsaid” policy and it may be sometimes necessary to live within the society, but if you want to be sincere and want to express your opinions you can always seek for polite ways.

Well, I think they're just careful about getting into 'controversial topics', really. Especially if they don't have a strong opinion about it either way (or aren't sure of the other person's opinion). They do give their opinions about normal things, though: if they think something is good, they'll be genuinely enthusiastic about it, but if they tilt their head a little, think a bit, and say 'yes.. it was good....' that means 'it was terrible'!! ha ha! :) More seriously; voicing strong opinions on serious subjects doesn't make for a harmonious, peaceful life.. that's how they see it, I guess.