[I would put a spoiler alert up here, but I'm not sure that this movie is actually spoilable, since it's all about a war that happened ~2500 years ago. That being said, if you don't know about the Battle of Thermopylae and have any intention of watching 300, you might not want to read this post.]
So, not to ape Eric or anything, but I too watched 300 last weekend. Overall, I wasn't too bothered by the historical inaccuracies. There were some definite omissions, of course, like the battle of Salamis. I guess I would have been happy to have a more complete and accurate picture of the surrounding history, but it was intended to be a recording of the battle of Thermopylae, and not of the entirety of the Greco-Persian War itself. By and large, from what I've read, the order of battle of Thermopylae itself was more or less true to the actual events, modulo the addition of fantastical animals and crazy mutant Persians.
The only real historical deviation that bothered me was the misrepresentation of the Greek army itself. The film gives the impression that the Greek contingent was basically 300 Spartans plus a bunch of mooks from the countryside. While it's true that there was a contingent of 300 Spartans present and that their contribution is the most remembered, the Greeks had a total of about 7,000 soldiers fending off the Persians. Which is, in and of itself, an amazing feat, and it would have been no less interesting a movie to show that battle instead.
A friend I saw it with made the point that the movie was effective at portraying the "emotional truth" of the battle, rather than the historical fact behind it, and I think there's some legitimacy to that argument. In reality, the Greeks of course fought in a phalanx formation and didn't charge willy-nilly into single-handed comment -- while I appreciated the brief representation of phalanx combat, it would have been boring as hell to watch what really would have amounted to a three-day-long shoving match. And of course the Persians didn't have 10-foot tall unarmored soldiers and the Immortals weren't hideously disfigured (and would Xerxes's army even have had elephants?). But those seemed to me like reasonable shorthand representations of the overwhelming sense of fear and intimidation that the Greeks would have felt when confronted by the overwhelming Persian army. Plus, they just looked cool.
Nah, I had only two real gripes. The first was that the movie was just plain boring. It was gorgeous, and the choreography was quite impressive, but there's only so many gruesome dismemberments and wave upon wave of endless combatants that I can see before the effect gets completely deadened. It was compounded by the nigh-invulnerability of the Spartan troops: it's hard to get emotionally involved in combat when none of the characters can die (which is sort of ironic, given that they do all die at the end, of course. Just not in the middle). The Spartans were quite impressive soldiers, of course, but they were hardly immortal, and were beaten in battle plenty of times (just ask Epaminondas about that...). I had the same problem with the big, gory fight scene at the end of Kill Bill Vol. 1 -- its over-the-top gore and violence was fun for a minute or so, and then it just got monotonous.
My second complaint had to do with the racial politics of the movie. I hold no truck with the comparison of the movie to current-day political issues; that just seems like knee-jerk criticisms (has anyone actually said this?). And certainly, told from the perspective of the Spartans, it makes some sense that the Persians are portrayed as uncultured brutes. But, given that Persians are essentially white and ethnically indistinguishable from Greeks, I'm a little confused by all the dark-skinned people serving in the Persian army. I mean, the Persian empire did extend into Egypt at this point, and I can imagine that they would have mercenaries from Africa, but I would think that the vast majority of the army would consist of troops from Asia Minor, who would be ethnically indistinguishable from the Greeks they were fighting. Am I seeing issues that weren't there?
And Eric, what was the ET reference? I appear to have missed it.