I love historical fiction. I cry with joy when I read a beautiful piece of historical fiction, something that makes me feel as though I've been dropped into the time period and am suddenly immersed in an era of horse-drawn carriages, sailing ships, beautiful dresses, and delightfully tight trousers or breeches on men.
What I HATE is inaccurate historical fiction. It drives me up the wall. There are several main things people royally fuck up, and it drives me absolutely bananas.
1) Titles. Not every woman is "Lady So-and-so". Lots of respectable women were "Mrs. So-and-so". There are a shit ton of titles, and you should get them right. Here's a handy Wikipedia link that explains titles for the nobility, the gentry, the commoners, and the clergy for the UK: http://en.wikipedia.org/wiki/Forms_of_address_in_the_United_Kingdom
Keep in mind, these titles have changed, and so you might want to do more research should you be writing pre-1700s.
2) The way the characters speak. You read a sentence like this: "forsooth, my lady, thou do not know what I ate for breakfast this morning you saucy rascal". HOLY. MOTHER. OF. GOD. No one spoke that way. Ever. Even Shakespeare, though he wrote things like "Shall I compare thee to a summer's day? Thou art more lovely and more temperate", never spoke like that. Don't believe me? Read his prefaces to his poetry. Nobody ever talked like that ever, so don't write your characters like that. "Miss Marlowe, you are a wicked girl and an impudent rascal, and I shall have you flogged" is fine, but "M'lady Marlowe, thou are a tyrannical trollop and you are totally bad, and I'm going to beat thee" is not.
3) Ships. Sailing ships. IF YOU CAN'T TELL A HEAD FROM A HALYARD, don't write about them. Don't write a pirate love story (oh dear God, what is it with people writing awful pirate romances?) without knowing anything about a ship. I don't think you need to name all the points of sail (irons, close hauled, close reach, beam reach, broad reach, running, in case you find yourself sexually attracted to ships the way I do) just to show off, but if you're writing about a captain on a ship, make sure he (or she, even though there's like a 0.0000001% chance of that happening), would say something like "lay aft here" instead of "come talk to me". Also, pirates, though they existed, hardly had the glamorous life depicted in fiction, so get a little more creative. Write about the navy men of the age of sail. They're just as hot, and their uniforms are to kill for. Just do your research, landlubber - the floor's called the deck, all ships are shes, and "hard a-larboard" is a direction, not a sex move.
4) Fantasy stories. They aren't historical fiction. Unless you've actually written a story that's about medieval history, it's not historical fiction, so get it out of that goddamned tag. Fantasy is great - Game of Thrones, anyone? - but it's not historical fiction. The only fantasy story I accept as historical fiction is the Temeraire series. And that's because that's two awesome things, dragons and the Napoleonic wars, combined.
It's easy to do research. Just look up the era you want, use Wikipedia or something, and look up clothes, titles, and the necessary props. Remember: effort is everything. If it's not perfect but you've tried your best, that's what counts.
And so help me, if I see one more story with a "Lady Cat", I'm going to punch myself in the face.