Japanese has a relatively small phoneme inventory, and luckily for English speakers it’s mostly a subset of the English inventory, but there are a few allophonic variations that we don’t have in English.

Pronunciation is not necessarily so simple, though: native speakers of every language employ a wide and relatively complicated set of phonological processes unconsciously. Here’s a partial list of the Japanese ones:

http://en.wikipedia.org/wiki/Japanese_phonology#Phonological...

Unless you’re unconsciously making these pronunciation changes on the fly, you’re going to have a discernible accent. I cringe when I hear English speakers of Japanese who haven’t internalized the simpler processes pronounce words like 人「ひと」as hee-toe and です as deh-sue. I’ve spoken Japanese for ten years and I still pick up something new every time I listen to a native speaker.

Regarding your other point, I always see people mention how complicated it is to learn to use the correct levels of politeness and honorifics in Japanese. But we encode largely the same information in English regarding relative social positions, familiarity with the conversational partner, etc. It’s really not any more complicated than it is in English, and in fact you could argue it’s less so because we primarily convey politeness with indirection and idiomatically. For example, if a waitress hasn’t brought your change at a restaurant after a few minutes and you say ‘Bring me my change now’ it would be incredibly rude. Instead you would usually say something along the lines of ‘Excuse me, I know you’re busy, but if it’s not too much trouble would you mind bringing our change over when you get a chance?’ You start by asking the waitress to excuse you for your imposition, go on to express that you understand the demands on her time, and then ask her (not command her) if she would mind bringing the change (not if she will actually bring it), when she has a chance to do so (not necessarily right this very second)—all despite the fact that you’re the customer and ostensibly she’s waiting on you. Depending on your tone of voice, the implication is that you really do want the change right now so you can leave but that whole song and dance conveys politeness and respect.

In any event, I have never had a problem speaking to practically everyone in Japanese using plain language. The only time I typically use polite language is with absolute strangers or people who seem in a bad mood, and even then I mostly speak in plain form and if your tone of voice is warm it serves to enact some closeness between you that people seem to appreciate. Very occasionally I’ve met a Japanese that seems to be a bit insulted if you speak too familiarly with them, but, honestly, I can usually chalk that up to that particular person just being a bit of a tight ass. If they persist in speaking to you in polite forms then you just switch back to the polite register to make them comfortable. Also, if you speak only in polite language then Japanese have a tendency to think that you only have a rudimentary understanding of the language. I have never used keigo, ever—if you’re in a business setting it might come in handy, but even Japanese themselves have to be instructed in how to use it when they enter corporate jobs. I can’t find the study now, but I was reading a paper that showed that native Japanese speakers have extremely warm attitudes towards foreigners who use keigo in business settings regardless of how proficient their use actually is or how many mistakes they make. If I remember correctly it was more or less irrelevant how well you used it—the Japanese had a high opinion of you simply for attempting to do so.