First of all, anyone who tells you this is historical fiction or SF/fantasy is lying. I read this book because I heard it was a borderline case, and because historical fiction with a hint of romance can be quite delightful (have I mentioned yet that I have a crush on Richard III?). But there's a lot of boinking in Outlander, enough to put it firmly in romance novel territory, although to be fair it's decorously written.
My quibble (aside from the one-time use of "least common denominator" by an 18th C. highlander, which may or may not be anachronistic but was at least awkward) was the sudden, unbelievable decision by Claire to abandon attempts at return to her own time and stay with her new husband. I'm sure furry Scots are quite appealing to some, but after waiting for her own husband's return from war for years, her choice seemed abrupt and even out of character.