I'm an Avatar series die-hard fan IRL, so the movie was really, really, really disappointing for me.
***Spoilers Below***
1) How Katara and Sokka (which in the movie is pronounced "soak-a" instead of "sock-a"...) find Aang (which is pronounced "ong" instead of "ayng" in the movie...) is wrong. In the series, Katara is yelling at Sokka and inadvertently waterbends Aang's sphere thingy out of the iceberg. Then, they break him out. They were not seal-something fishing and found a bubble under a lake.
2) After like a minute of the movie, Aang is captured by Zuko (who demands that the village bring out the elderly). On the ship, Iroh (pronounced ee-row instead of eye-row) gives Aang a test to find out if he's the Avatar. This never happened in the series. The monks tested Aang while he was at the Southern Air Temple in the series, and while on the ship, Aang was held prisoner but fought Zuko and escaped. While attempting to flee the ship, Zuko grabs a hold of Aang's foot and drags him back down. After being knocked into the water, Aang's Avatar state is triggered and he destroys the ship. He then flies and meets up with Katara and Sokka. In the movie, he just simply flies away. No Avatar state, no ship-destroying, no fighting, nothing.
3) In the movie, Fire Benders need a source of fire in order to bend (WTF?). This is totally made up. In the show, Fire Benders create their own fire with their own energy, which has been corrupted in a way, so that they bend using anger and rage and whatnot. But in this movie, they need a close-by torch, campfire, etc. Later in the movie, Iroh demonstrates that he is able to bend fire without a source, which the other Fire Benders are amazed at. "Oo, Iroh is bending fire from nothing! Wooow! I wish we could do that!" Ugh. >.>
4) Aang discovers the Avatar statues at the Southern Air Temple in the series. In the movie, he goes to the Northern Air Temple where he meets some unnamed guy (I'm assuming is the Mechanist, but it's never told) that tells him his past life was Roku (in the series, Aang knows this himself by staring at Roku's statue in the SOUTHERN Air Temple), then betrays him for money after blaming his poverty on Aang, and Aang is captured (and later broken out by the Blue Spirit [aka Zuko] - at least they got that right). And another thing, Roku is an actual person in the series who rides a dragon. In the movie, he is the dragon. ????
5) In the series, Monk Gyatso was an elderly, small, Asian man. In the movie, he's a young, well-built, African-American man. Now, I am NOT being racist. I don't mind if they used an African-American for Gyatso because he was a great fit character-wise and whatnot. But at least get an older man!!! This actor isn't even CLOSE to what Monk Gyatso looks like or really even acts like in the series.
6) Fire Lord Ozai is a relatively-young, attractive, power-hungry man in the series. In the movie...well first off, we're not even supposed to see Ozai's face until book 3. In the movie, Ozai is revealed to be an older, sophisticated, calm man with a slight accent. Again, not even close to the Ozai in the series. Oh and evidently, "Grandma" (instead of Gran-Gran) explains to the Katara and Sokka that the Fire Nation's goal is to live without "The Spirits'" guidance. Shyamalan brings up this whole "spirits" thing in the movie. Yes, Aang is the bridge between reality and the spirit world, but it's not as in-depth as Shyamalan tries to make it out to be. The Fire Nation isn't rebelling against these Spirits, they just want to wipe out the rest of the world so they can be the dominant (or sole) nation in the world.
7) In the series, the gang stops at a small Earth Kingdom village that is overtaken by Fire Nation soldiers who forbid Earth Bending. Katara meets Haru. After attempting to help someone, Haru gets arrested, so Katara lets herself get arrested so that Aang and Sokka can figure out where they are being taken. The prison is a giant ship on the sea, where there is no earth. They devise a plan to shoot coal up this big chute for the Earth Benders to bend, and overtake the Fire Nation ship and be free. Well, in the movie, the kids stop at the village and meet what I assume is Haru (in the series is about 14 or 15, Katara kinda has a crush on him; in the movie, he's like...7 and never speaks). They get to the village where the Fire Nation is camping out. No one gets arrested here, though. Aang tells the Earth Benders that earth is all around them and that all they need to do is use it. Oh also, there's a statue of Avatar Kioshi in this village (um... so we just completely skipped over Kioshi Island, then, and stuck it in a completely different town?). Series - Katara's speech on metal ship + coal = revolt. Movie - Aang's speech in town + earth = revolt. So yeah, completely different. Oh, and someone in the town hands Katara her water scroll, rather than in the series where she steals it from pirates.
8) I could keep going on, but it just keeps getting worse from here. They never meet Jet nor make any mention of him, they never go to the Fire Temple on the Soltice to learn of Sozin's Comet, they never meet Jeong Jeong. In the series, the comet will be arriving by summer's end. In the movie, Ozai tells Azula (who is like eight years old) that the comet is three years away (pulled that number out of your butt, eh Shyamalan?) and that he needs her to stop the Avatar or something.....I kind of wasn't listening by that point.
The acting was pretty bad (Sokka had not one drop of humor or excitement at all, Aang was this mopey emo kid, and Katara had no personality whatsoever), the special effects weren't all that special. Ok, bending is an extension of the bender's person. A wave of your arm creates a devastating air blast. But in the movie, they do these big long dance sequences for just one little puff of air. As I read in one review, "sequences of interpretive dance with bits of the elements tossed in". The story jumped from one place to another with barely any explanation. I love the show and know it pretty darned well, and even I was confused watching this movie. Like, who the heck is that, and where are they???
Oh, OH OHHHH, and in the Northern Water Tribe at the North Pole, you know the final battle where Aang joins with the Ocean Spirit to make this giant water creature thing that destroys the fleet? In the movie, that doesn't happen. He makes a big wave, hangs it over the ships all threatening-like, and they turn around and leave. What??? It was so anti-climactic. And rather than the Ocean Spirit killing Zhao, a random group of Water Benders do.
It's like, I don't expect Shyamalan to incorporate every single episode into the movie. What I do expect, though, is to include the major points (a la Solstice, Jeong Jeong, Jet, Kioshi Island, etc.). And the points that are included should at least be accurate. But it's like...he took this fantastic series, crapped it out, and called it a movie. Everything is out of place and the whole thing just stinks.
Now, I understand there will be people who like it. Perhaps those who aren't familiar with the series, or even people who accept that live-action films are often loosely based off of their source material. But for me, an Avatar junkie, I expected a lot, LOT more than this garbage. My husband and I wanted to walk out after the first few minutes, but stuck around. I will not be seeing Book 2 and Book 3 (lord, I hate to see what they'll do to Toph, and they probably won't even include Appa's kidnapping). Biggest waste of $20 in a long time.

I will say, however, that Zuko's character played by Dev Patel (correct me on that if I'm wrong) does a pretty darned good job. He captured Zuko's personality well and made him believeable. I would have liked the scar to be a bit more obvious (he didn't get scratched by a kitten, he got freaking burned), but I can accept that. He did a good job.
/end rant