Well, at the risk of sounding snarky: I think that’s more likely due to the fact that the thread talks about two movies which have almost nothing in common other than the type of filmhouse they tend to be shown in. And to be honest the tone of your OP is more than a little snobby: there’s nothing inherently good about indie movies or inherently bad about mainstream ones. Sure, there’s a ton of crap coming out, but that’s always the case. There’s some great stuff coming out, too.
Sure, the two movies from the OP have appeal for adults. I would say that Finding Nemo does, too – it’s about the relationship between fathers and sons, and about learning to live without anxiety. And both of the movies from the OP are very good, but neither was perfect, and neither was particularly groundbreaking. My point is that there’s not much use in dividing movies into “mainstream” and “independent;” I’d rather judge them on their real merits.
Anyway, American Splendor I liked a good bit at the time, but have already forgotten almost everything about it. They get high marks for their technique of interpreting the comic to film, but in the end it’s just a very abridged adaptation of the comic.
Lost in Translation I liked a lot better; I thought it was outstanding. It spends a good amount of time navel-gazing, and in the end you realize it’s just a fairly straightforward love story and not nearly as profound as it seems while you’re watching it. Plus, as has already been pointed out on here, most of the secondary characters were pretty one-dimensional, and a lot of the humor was based on tired old obvious stereotypes. (Japanese people say L instead of R! Hollywood movie types are really superficial and shallow!)
But none of that really matters, because the focus of the movie is so well done. The dialogue is what impressed me the most – no speeches, no exposition, no sudden epiphanies that prompt characters to pour out everything they’re feeling. This is the way that real people really talk in the real world.
And Murray and Johansen’s performances were just dead-on perfect. I really hope Murray gets recognition for this one because it’s his best performance ever (and I thought he was stellar in Rushmore). He lets his personality through, of course – for him not to do that would be just a waste – but never drops out of character. And he shows a level of subtlety and control I never would’ve thought him capable of. And again as already mentioned, Johansen had the tougher job in many respects, but handled it perfectly. The scene where she’s trying to explain herself on the phone to a friend but can’t stop crying, is moving just because it’s so underplayed.
The movie also does a great job of showing just how weird and foreign Tokyo and Kyoto seem to outsiders. Kyoto is peaceful and tranquil but still extremely alien to Americans. Tokyo has so many things in common with every other big city on the planet, and borrows liberally from American and European culture, but remains distinctly Japanese. The movie hits the cliches – karaoke bars, bullet trains, Mt. Fuji, etc. – but captures the feeling of the places perfectly. At least, the feeling for visitors – I have no idea what residents would think of the movie.