I've read a lot a reviews of The Hobbit: The Desolation of Smaug the last couple of days, and every single one of them fixates on the characters of Legolas & Tauriel. It is usually an interesting analysis, for most reviewers seem to think that Orlando Bloom & especially Evangeline Lilly did a good job, but nearly all reviewers decry the fact that "they're not supposed to be there." Legolas first appears in The Fellowship of the Ring, not in The Hobbit, a book that takes place many years before his introduction at the Council of Elrond. Tauriel is, admittedly, not a character from J.R.R. Tolkien's legendarium. Tauriel was an invention of Peter Jackson and the other writers.
I disagree with any assertion that Legolas does not belong in The Hobbit. Legolas is the son of the Elvenking Thuanduil, king of the Mirkwood. That makes Legolas the prince of the Mirkwood, and very likely the captain of the military forces. Is Legolas old enough to be in The Hobbit? Of course he is. Tolkien never gives us a birthdate for Legolas, but there are many hints that he is well over a thousand years old, very common for an elf. The events of The Hobbit happen almost precisely 78 years before the Council of Elrond. When you look at the facts, it would be far more unlikely for the elves of Mirkwood to be major players in The Hobbit (they are) without Legolas taking center stage. In the book The Hobbit, none of the Mirkwood elves are named, not even Thuanduil, who merely goes by the title Elvenking. If they wanted to be true to the books, they should have included Legolas in the movie without ever mentioning his name; that would have been interesting.
Regarding Tauriel, I freely admit that she does not belong in the story. Characters like Radagast the Brown were Tolkien characters who do not appear in The Hobbit, but whose stories come from the peripheral Tolkien works. In the extended cut of The Hobbit: An Unexpected Journey, we are introduced to another such character: the Old Took, as well as his daughter Belladonna, who was Bilbo's mother. Neither of those characters are in The Hobbit either. I am totally OK with Tauriel (assuming that her character works in the movie, as we will see later this week). The fact is that The Hobbit is ... well ... if I wasn't a pastor, I might use the vulgar term "sausage fest." There are exactly zero women in the book The Hobbit. The only one mentioned is the aforementioned Belladonna, who does not appear in the narrative. However, the elves of Mirkwood do apprehend The Company of Thorin Oakenshield, and presumedly, at least one of those elves is female. It is good to include a female in the story. If Tauriel was, for instance, the grandmother of Aragorn or Boromir, just shoved into the movie as a needless tie to The Lord of the Rings, then I would have a significant problem with the character. However, since elves are supposed to be there anyway, this is not a big deal at all.
Indeed, putting the elves of Lothlorien in the Battle of Helm's Deep is a much, much bigger departure from the books, and we've long since gotten over that one.