How do you know a show is good? How exactly is it that you separate the shows you love from the ones you hate?
Is it three-dimensional characters and detailed characterization? Is it if it has a unique premise, or if the story has proceeded in a unique way? Is it good pacing? Is it good music? Is it good animation? Is it not being moe? Because K-On! fails on pretty much every level in all those categories.
The characters are one-dimensional moe archetypes. They really don't have any new twists to them that make them unique. They're characters that you've seen in dozens of shows before, and they were probably done better in those shows. None of the characters could really be said to go through that much of an arc; while they do change a bit throughout the show it never really feels like they've become a different person, or even really matured significantly.
The cast can also come across as almost carbon copies of each other at times. Ritsu and Yui are incredibly similar, and if you judged by their personalities, Mio and Azusa are essentially twins. And the fifth member of the band, Azusa, essentially comes in near the end as sequel-bait, doesn't really accomplish that much, and could have easily been cut from the show entirely.
The story is equally as dry and cliche. We've seen outright identical storylines in anime like Beck, and it was done better there. There are no new elements added- from the beginning, you can accurately predict almost exactly where the show is going to be headed from beginning to end. The show tends to drag at points- the episode where Yui buys a guitar, for instance, could have been cut to ten minutes, and several episodes could have a good five minutes or so shaved off.
One final huge flaw is the nature of the comedy- often times the jokes are extremely repetitive. Yui is incredibly unfocused and immature, and her sister is the opposite. Expect this to be the punchline of approximately eight to nine jokes throughout the series, often essentially told in the same way. Tsumugi is extremely rich. Expect there to have been about twenty jokes about how rich she is to have been made by the time the series ends.
If you're watching for the music, don't expect to hear that much played. The show doesn't revolve around the band, really. It revolves around the girls working together in creating the band- which involves an incredibly large amount of slacking off and simply sitting around doing nothing. There are three episodes where the girls actually go on stage and play music, in a thirteen episode show. That's a sign that there's a lot of fluff packed in there.
The music is decent, but not particularly memorable. I certainly wouldn't be found humming it in the shower or buying a soundtrack. I liked the OP and ED, but they weren't anything special. They were just base J-Pop with a nice beat that didn't outstay their welcome. The same is true for the songs the band plays, really. They aren't bad songs, but they certainly aren't great ones either. Which is not really a good sign when the show is about girls in a band. And the rest of the music in the show are pieces you forget the moment they stop being played.
The animation is pretty inconsistent. The frame rate is typically decent, though the camera often utilizes weird angles. The characters builds all look disturbingly similar, but not similar enough to be considered identical. And, of course, they have been created using the traditional moe art style we've seen hundreds of times before. It constantly reminds the viewer that this is a moe show for fans of moe, and that you're an idiot if you ever forget it.
Overall the backgrounds are pretty, though they just have this... two-dimensional feel to them. They don't feel that vibrant to me. Oh, and there's a good chunk of fanservice for the sake of fanservice. Because everybody loves that.
The main aspect that I will compliment is the voice acting. A lot of fans have bashed the English dub, but I thought it was great. The VAs play off each other extremely well, and do know how to actually act- something that is often neglected when discussing VA work. The cast is full of energic performances by people who want to have fun with their roles. Laura Bailey is pretty clearly phoning it in, but she's good enough that she can still produce a quality performance.
Oh, did I forget to mention? K-On! is a good show. Anybody who doesn't outright despise moe should give it a chance. How can I say that after I wrote the above? Simple. I left out the most important criterium to determining whether or not a show is good: did I enjoy watching it? Because I got a huge kick out of watching K-On!.
Every episode was great in its own way, despite sometimes feeling like they could have been a bit shorter. I laughed at the funny moments, actually got involved in a couple of the emotional moments, and the end brought both a smile to my face and a tear to my eye. Yeah, I'm a sap when it comes to emotional endings. Sue me.
The characters all actually complimented each other excellently. You really thought that this group of characters- whether they're archetypes or not- could actually be friends, which is more than I can say for plenty of the slice-of-life anime out there. The entire show is reliant on character humor, and it works really well despite being repetitive. And although you could cut Azusa from the show, I really don't want Azusa cut from the show. She's interesting, and actually provides a fresh straight man, right when Mio's odd-man-out shtick was getting old.
The story is cliche, but it doesn't try to be anything else. It isn't trying to be groundbreaking or anything new- it's trying to have fun with the established tropes. That's really why the plot works- it doesn't take itself seriously. This show knows that it's cliche, it knows that it's not really adding much new; it doesn't care. The writer(s) just wanted to have a good time, and that's exactly why the viewer can have a good time too.
K-On! is flawed in various ways, but you simply can't not have fun while watching it. If I was entertained by a show, it succeeded. K-On! entertained me. Therefore, K-On! succeeded. QED.