I recently finished the anime Sola, which, despite flaws, I managed to enjoy greatly. Opinions, however, vary greatly, and one common complaint among critics of the series is that it borrows too heavily from its lead writer Naoki Hisaya's previous work, Kanon. A prime example of such arguments comes in the form of this satirical summary/comparison piece by a currently unknown author:

The clear implication is that Sola is little more than a carbon-copy of Kanon; a damning indictment to be sure, particularly in light of the common author, but is the argumentation solid? A closer look reveals that the comparison rests heavily on overstating broad similarities, exaggerating liberally and cherry-picking examples .

Let's start from the top. The first paragraph establishes that both series involve a guy interacting with girls from his (unknown) past, which is absolutely true, and, in my opinion, the primary similarity between the two. Yet a common plot element, if a central one, is not enough to substantiate a claim of (self-)plagiarism. One might as well argue that AIR (another work of Hisaya's) is a copy of Kanon as well, as it also involves a guy-girl relationship rooted in a remote past. (Granted, AIR only features one girl from the past, against the four of Kanon, but if the author can reduce these four to two for the sake of argument, my little creative liberty can surely be excused as well.) The simplistic analysis employed by the author obfuscates the quite distinct functions of Yuuichi and Yorito's amnesia in the plot--Yuuichi is conscious of his amnesia, and it serves as a central point of tension and foreshadowing through much of the story, while in Yorito's case it is played as a more or less sudden revelation--to say nothing of the divergent circumstances by which the two characters forgot their pasts in the first place.
Note as well that the parallel between the Yuuichi-Ayu-Nayuki triangle and the Yorito-Matsuri-Aono triangle is described only as a "shared past," most likely because going into any further detail would blow the lid off the comparison--Ayu and Nayuki never even knew each other in the past, whereas the dynamics between Matsuri and Aono are the driving force of Sola's story. The author concludes the first paragraph by pointing out the mutual use of the "X is actually dead" trope, certainly a salient comparison, except that--look closely--Yorito has suddenly switched from being a Yuuichi analogue to being an Ayu analogue. All perverted crossover ideas aside, be prepared, as this is far from the first time the author will play fast-and-loose in ascribing roles.

Take, for instance, the author's next paragraph. The author's rationale for labeling Matsuri as an Ayu analogue is, first, that both have strange mannerisms and favorite food/drink items, and neither is quite an ordinary human. Valid, but overly broad; characters with any of those traits, and frequently various combinations thereof, are a dime a dozen in the wild world of anime. The claim that both are "secretive" is a bit more problematic. Yes, initially neither is completely forthright about her background with her respective protagonist. But Ayu has no choice, being largely ignorant of this background, and in Matsuri's case spilling the beans would be a massively consequential decision, one which she reaches only hesitantly. Considering, furthermore, that she is rather candid about her identity as a Yaka, "secretive" seems an exceptionally poor word choice to describe Matsuri. Conspicuously missing is any mention of the girls' personalities beyond that they are "eccentric" and (ostensibly) "secretive," probably because it would be devastating to any attempt to liken the two.
The parallel between the girls' antagonists is even more of a stretch. Ayu is hardly "always" on the run from the taiyaki baker; she is seen fleeing him no more than a couple times, after which he fades into irrelevance, never having been a character in the story so much as a prop; his name is never given, his face barely shown. Takeshi, the bearded man, may receive his character development grievously late in Sola, but even he isn't that badly off. However, he's not out of the woods yet, as the piece gravely misstates his motivations. It is unknown, not even strongly implied, that Matsuri was actually responsible for turning Mayuko into a Yaka (and even if she was, it was only after Mayuko was murdered by the robbers who would make a far more sensible target for Takeshi's anger,) and Takeshi is unambiguously seeking a sacrifice, not revenge. It is difficult to say that Matsuri and Ayu's commonalities exceed rather widespread idiosyncracies and broadly similar roles in the plot.

The author continues by addressing Aono, who is suddenly an analogue of a different Kanon girl (Mai.) The author is correct to point out that both girls are reticent and good in combat; however, as in the case of Ayu and Matsuri, these are very common traits for anime characters, and to ascribe sameness on the sole basis of sharing these traits is, at the least, highly questionable. The claim that both girls are "protective" of their respective protagonists is equally dubious. Mai is protective of Yuuichi in the sense that she cares about him and would prefer that he didn't get his ass killed by demons. Aono is protective of Yorito in that she is all-consumingly obsessed with him and will attack anyone who tries to come between him and her. As a result of their differing means of caring about their beaus, Mai and Aono play radically different parts in their respective stories. Both girls are protective, but only in the loosest sense of the word "protective."
Granted, Aono resembles Mai more than Matsuri resembles Ayu. Yet, if they are cast from the same mold, the two characters are used in very different ways, in a bit of an inversion of the Matsuri-Ayu relationship.

So what happens next, you ask? The final paragraph actually presents a fairly accurate summary of the Big Plot Twist in both series (aside from the first sentence--in both cases the revelation that the girl can no longer see the guy comes prior to the self-inflicted exile.) Once again, though, the author plays a frenetic game of mix-and-match pigeonholing Sola characters into Kanon roles. More problematically, two major things are missing from the end of the author's analysis: 1) what happens next, i.e. the actual ending, and 2) the reason why all that crap happened in the first place. Once again, these errors of omission may not be accidents, because in those pieces of the story is where Sola and Kanon are at variance. The endings and the plot-furthering factors of the respective stories speak to a greater fundamental distinction: Kanon is, at its heart, a story about the power of miracles, while Sola is better characterized as a story about the importance of accepting tragedy and moving on.

It is undeniable that Sola was informed by Kanon, and borrows more than a few elements of story. Yet to call one an uninspired facsimile of the other is to grossly overstate similarities while ignoring manifold differences. The methods the author of the piece has used to do so can be used to make such a claim about nearly any two works of fiction, and to demonstrate, I shall now compare the hit anime The Melancholy of Haruhi Suzumiya and J.R.R. Tolkien's magnum opus The Lord of the Rings...

The Lord of the Rings Melancholy of Haruhi Suzumiya is the story of Frodo Haruhi, a brown-haired, somewhat eccentric guy girl with an inexplicable fascination with the exotic. His Her fantasies are realized when his her idyllic life is interrupted by the receipt of the revelation that she has terrible power. Due to the great danger this power poses, five three extraordinary people are sent to watch over Frodo Haruhi. Together with Frodo's Haruhi's existing friends, they have many adventures, such as staying with Tom Bombadil Itsuki's relatives and fighting Nazguls the Computer Research Society. Ultimately, just when Frodo's Haruhi's power threatens to consume him her and all seems lost, Gollum Kyon saves the day with a bite kiss.
Ergo, Lord of the Rings = Haruhi. Or not.

In sum, there are three questions one should take away from this debate:

Does Sola draw on Kanon? Yes.
Is Sola a cheap ripoff of Kanon? Hardly.
Is Sola, warts and all, worth watching? Absolutely--whether or not you have already seen Kanon.

p.s. matsuri is hot