It's actually rather amusing to watch the frothing at the mouth that starts up any time someone dares mention Heinlein in a favorable light without the cop out backhand "man of his time" excuse. See, that particular argument is horribly misused most of the time. It's used to dismiss the visionary aspects of someone's achievements because they - duh - had some of the blind spots of their era. Instead, it should be a recognition of how extraordinary the achievements are in view of the time when they occurred.
Of course, that's not what causes the frothing. It's that Heinlein forces you to think. My experience is that very few people willingly examine their assumptions about how the world works. The folks here at MGC are something of an exception for a number of reasons, not least of which is that of the six of us, half of us have uprooted and moved to another country. Another reason is even simpler - much of the debate about Heinlein, genre fiction - and even Western culture - is US-centered. Amanda is the only regular MGC poster who is 'Merkin by birth. Sarah and I adopted the place, Dave is South African recently relocated to Australia, and Rowena and Chris are Aussies. Even Amanda isn't 'typical' 'Merkin - she's traveled to parts of the world most USians couldn't find on a map, and even learned other languages. Our readers are a mixed bunch, too - which we appreciate.
Why do I mention this? Because someone who's moved country, or who does a lot of work with people from a different country, is in a situation where their assumptions about how the world operates get questioned or they get smacked in the face with evidence to the contrary.
This is of course a staple of fiction, forcing a character to question and change their world view, but it's an uncomfortable thing when it happens to you. Most people get very aggressive-defensive when their basic ideas are challenged - and funnily enough that's the exact tone of the anti-Heinlein frothing.
And that, dear reader, is the reason why there is a proverb to the effect that the prophet is without honor in his own country. He's - pretty much by definition - challenging people to confront their deepest assumptions. If people can't just dismiss him, they get aggressive and try to belittle him by any means available.
Heinlein - obviously - is one of SF and fantasy's prophets. Pratchett is another. Who would you nominate?