Among all the anime that comes out every season, the majority of them are adaptations. They mostly fall under one category of adaptation, and fall to the same traps, but there are a few others that do things differently for better or for worse.
The "Typical" One:
Notable Example: Just pick an adaptation. It'll probably fit in here.
I'd probably say about 95% of anime adaptations will fit in here. This is the kind of adaptation you see absolutely everywhere. In this kind of adaptation, the anime will adapt as faithfully as it can everything that happens in the source material up to a certain point (essentially as far as 12/24 episodes take them). After that, most of them will end here and be forgotten. The hope is that it does well enough in order for it to live on and continue the adaptation. If it succeeds in doing this, it may go onto the next type if it can finish adapting the source material. Otherwise, it remains in this category.
The big problem with these adaptations is that you're only ever getting part of a story. You get a mere taste/sample of something and nothing more. This either gets you spurred into trying to find the source material, or makes you excited, but then eventually fades away as time goes on.
What this results in is anime that are just okay. They look like they can have great potential when done well, but without finishing the story they start, they leave you with a feeling of incompleteness. This kind of leads to anime that doesn't seem to go anywhere, or doesn't use their setting, etc, to their full advantage. Perhaps in the source material, things get sorted out, but in just the anime, you would never know that.
But, it's understandable why studios do this. It's all about trying to strike oil and find a gem which they can milk for easy money. Due to the alternatives being much more difficult, this is a reason why there are so many adaptations each season.
The "Complete" One:
Notable Example: Clannad
The ones in these categories are typically ones that were in the previous category, but struck gold and resonated enough with the audience to get fully adapted. Although there are cases where it manages to do it in a single go (Steins;Gate).
The fact that they can tell a story from beginning to end already tends to put these ones on a higher scale than the "typical" category. These ones have a coherent start, middle and end, and function like a typical story, unlike many odd arcs like other adaptations do. In other words, they feel complete.
In an ideal world, every studio would probably make adaptations like these as opposed to having to make "typical" ones, and not continuing them because it didn't work out as planned.
While these still tend not to live up to their source material, as due to the tendency to have to cut out details, they can be an acceptable alternative to people who don't want to/can't experience the source material, but still want to know the story of this series.
The "Bad" One:
Notable Example: Fate/Stay Night
Everyone once in a blue moon, a studio tries to take the reins of a series and try to do something to it either by adding to the plot, taking things away from the plot, or changing how information is presented, etc. This often leads to disaster. Luckily these have for the most part stopped appearing.
A bad adaptation can destroy appeal in a series and cause people to form negative associations with it, even if the source material is good. Fate/Stay Night is a great VN, with a great story. The anime tried to shoehorn in a bunch of different parts from different routes, which lead to an incoherent mess.
It's really sad for fans of the series when they get an adaptation like this, as they often expect to at least get something resembling the story they love, and when the receive an abomination of their series, it's hard not to be sad.
The "KyoAni" One:
Notable Example: Kyoukai no Kanata
The studio KyoAni recently started to do their own interesting take on an adaptation which I haven't seen else attempt, and is quite interesting.
When they adapt a series, for the first 2-3 episodes, they may follow the source material, but it quickly diverges and goes on a completely different route. Character's that didn't exist in the source, plotlines that never happened, all of these things being to show up in the anime that never occurred in the source.
They essentially take the characters and settings from a series, and build their own story with these resources. What this allows them to do is tell their own story, with clear climaxes, beginnings, and ends.
They get to avoid the pratfall of the "typical" adaptation and finish the story they start. In the case where it earns another season, they can always make more content, as they're the one who created the story lines in the first place!
It risks the original fanbase not liking the direction the anime takes, as it may seem off-putting to be familiar with some characters and see them being kind of reimagined of sorts from the mind of a different person. However, this is never a point of contention to the western anime viewer, as they generally have no association with the original work.
This requires a lot of effort from the studio attempting to do this, and it may only work because KyoAni is KyoAni, and they can consistently deliver good works, but it's an interesting strategy nonetheless. I personally appreciate them more than the "typical" adaptation, but that just may be the KyoAni fanboy in me talking.
What are your guys's thoughts on this? Is there another kind I may not have thought of?