If a manga or light novel series is succesful enough, for whatever reason, it’ll probably end up being adapted into an anime. Vice versa goes for original anime series. The fans of the original work are almost always going to check out the adpatation in order to see their favourite series brought to life, and the fans of an adaptation may go back to see how the original work portrayed everything they saw in the adaptation. I think that fans are more likely to be disappointed with the changes or differences in the latter adaptation they’ve seen.
It has to be said that with adaptations some creators/studios/mangaka have their own view that they want to get across. For example, with the anime Bokurano, the director of the anime said that he didn’t like the storyline of the manga and had decided to change it. As such, I’m guessing that many of the manga fans would’ve been annoyed that he’d taken such liberties with the plot. However, most of those who hadn’t read the manga beforehand (myself included) thoroughly enjoyed the show anyway. Does this mean that the manga fans weren’t justified in their irritation? No, simply that the anime in and of itself was a good show that, though it didn’t do the same things as the manga did, had its own merits that may not have appealed to the manga readers who had an attachment to the original work that they read.
I believe that if you really like the first version that you see (say something you’d give an 8-10/10) then you’re more likely to be disappointed in the differences in the second version that you see. You’ve come to appreciate the all the elements of the first version, so you’ll appreciate it more. It’s kind of like the subs v dubs debate. The voice you hear is the voice that you come to associate with the character, so hearing something different may feel awkward even if the voice actor is relatively good. This even relates to adaptations of manga: you may have had a certain voice for a character in your favourite manga series and when that series is adapted you hear something that you don’t really feel fits the character that you’ve gotten to know. When you’re awaiting an adaptation you’re probably waiting to see your own individual vision of that world brought to life, and when that vision isn’t what you had expected, you’re likely to be disappointed.
Of course, this is never set in stone. Sometimes there are flaws in the first version that you’ve seen that are addressed in the second ( i.e changes in personalities of characters). There’s also the medium to consider. Most anime only go on for a year or less, whereas manga and light novels have many years (even decades) to develop their stories and characters sufficiently. As such, the “disappointment” factor may be more in relation to manga/light novel to anime.
In any case, rather than just saying “difference is bad, faithfulness is good” I think I’ll just say that there are both things to appreciate and things to be disappointed in whenever a series is adapted. After all variety is the spice of life anime, right?
Grand Punk Railroad 
I’ve always agreed with this point of view. Nostalgia is a powerful thing, even when the difference in time is relatively short. Whatever you saw first holds a more special place in your heart and all that jazz. Mostly. Naturally, it’s best to be able to judge things independently of source material or adaptation, and it’s one of the reasons I’ve continually put off reading the FMA manga — if this remake is supposed to be more faithful, great, but I’d like to see it for what it is standalone (minus inevitable comparisons to the first anime since I’ve already seen it).
I find myself to be pretty purist about a lot of things though I really try not to be. (Best example: Harry Potter movies + Death Note anime.) Still, there are a lot of adaptations that I find myself really, really liking — maybe not more than the source, but still — like the Death Note live action movies, which are hugely different from the manga, but I love both a lot.
I usually find that I almost always prefer the original, which is why I usually try to check out adaptions first. To avoid disappointment. I think it’s because there’s usually (but of course, not always) more content and development.
Usually I think that the original creator does the best job with the story. As long as they really put effort in and like their series, they’re usually the one that knows it best. Especially with characters, I find beloved character too often being OOC in adaptions so that they can fit whatever the story needs for the moment. I don’t like that at all, so that alone may make an adaptation fail for me
But I think you’re basically right, when the anime and manga are both the same length, but just…different (and if the characters are more or less as likable). Though, I think it depends a little on how much you like the first thing you see too. There would be something that bothered you that gets fixed in the other version…
Ugh…basically I guess I should say that I think it’s complicated. Too many factors for a general rule.
You’ve definitely got a point, and the mention of subs vs dubs reminded me of the first time I played Halo (it’s relevant, somehow). Long story short, I was already used to Japanese voices in video games from JRPGs, and back then, I pretty much expected myself to always prefer Japanese voice acting over English. But when I played a Western FPS, it felt perfectly natural for it to be voiced in English. Does this mean that Master Chief speaking Japanese would sound bad? Probably not, but I heard the English version first, so it’ll sound awkward anyhow.
Bottom line, I loved the ef anime since I had nothing to compare it to: and if I ever watch Death Note, having only read the manga, I might be disappointed. It explains many of the debates that go on between fans of a particular franchise’s anime and manga.
It’s a consequence of the “source is sacred” mentality. In the majority of circumstances, fans of the source are probably right that the original is better, because there’s less pressures on the creators to compromise it in order to make it more popular. The problem is, what most people will see as “ok, nothing brilliant, entertaining for what it was”, source fans will bemoan as an “absolute travesty”, because they’ll inevitably compare it with a source that they’ve grown attached with, nuances and all.
I really can’t stand the “loyalty is everything” mentality when it comes to adaptations. It’s like saying that it’s impossible to do better than the source, therefore there’s no point in even trying. My ass!
>>>first version, so you’ll appreciate it more. It’s kind of like the subs v dubs debate. The voice you hear is the voice that you come to associate with the character, so hearing something different may feel awkward even if the voice actor is relatively good.<<<
And sometimes the original is just better. Case in point, English Ayu couldn’t pull off a proper ~Uguu.
Well, certainly if you think something approaches perfection, any deviation is likely to be bad rather than good. This is statistically true (there are more microstates that lead to diminishing quality than there are that lead to an increase in quality.)
On the other hand I think it embodies the true creative spirit to not rest on one’s laurels and to want to try to improve and tinker, even with the nearly perfect.
@Kiri
I’d think that it’s be better if someone were able to judge a show fairly, even if they had seen the source material.
I disliked the Death Note adaptations mostly because I think the acting of everyone except the two leads wasn’t up to par.
@FuyuMaiden
I agree that the crators know their series best, but it’s always great to see someone who’s capable of adapting that story put their own spin on it. Sometimes the eyes of the creator can’t see things from the perspective of a fan.
@ETERNAL
Both Japanese and English voice acting have their positives and negatives, it’s more the transition of one to another that leaves a lot to be desired.
And you should definitely watch the Death Note anime, IMO. I’ve never read the manga, but it should at least be good in that you can see some of the best moments from the manga animated.
@Sorrow-kun
But what about those shows that are fantastic I their own right, like Death Note? I don’t remember there being much haet from the manga readers when Death Note aired except for the last couple of episdoes, and even then that was only a couple of blemishes (for the manga fans at least) on an otehrwise great show.
@Index Librorum Prohibitorum
Then they probably should’ve eliminated it altogether in the English version. The Japaese have waaaay too many idioms in their language.
@moritheil
It’s all subjective anyway, so that that could be considered to be near perfect may not be so in light of what’s better in an adaptation.
[…] more, being ever so slightly more of a “character” man myself (and the fact that the first girl wins) but reading the manga as well means I get my grand and engaging plot as […]