After seeing the Critic's review, I must agree. Why wasn't the blind guy healed? Absolutely everything else was fixed!
The only thing that makes any sense as an answer to this question is that maybe they didn't want kids to think that blindness has a magical cure. Note that everything else repaired by the sequence was a magic-based condition, and therefore has no correlation to the world in which we live, but blindness really does exist and really can be caused by a traumatic head injury such as Garret suffered. The Powers That Be probably had some concerns that if they fixed Garret's blindness by magic, young kids might get the wrong idea. Not saying they were right to have these concerns, necessarily (I think kids tend to be smarter in general than they're given credit for being), but it may have come down to being safe rather than sorry.
Also, since Garrett was treated as an outcast because of his blindness, they may have not wanted kids to think that Garrett had to be cured in order for the knights to accept him. Rather, he overcame the stigma of his disability, and convinced them that he could be a fully efficient knight regardless. Not having him be cured at the end suits the Aesop of the film, but unfortunately doesn't make a lot of sense in context with the story.
Um, we don't actually get any evidence that Garrett is still blind when Ruber dies/kills himself (I can't decide which applies), so maybe his blindness was cured, but we weren't shown it as the creators realised how unbelieveable it was (leading to the Critic's question). Certainly, from what I recall (I've not seen it for about two years, although I did watch the Critic's review and agree that, in hindsight, there were more questions raised by the film than answered), it looks like his vision returns, as he seems able to recognise Kayley just before they were knighted, as well as possibly recognising Merlin (a person he won't have heard for at least ten years, which means he would have needed perfect recollection to realise who it was otherwise).
The final scene doesn't go out of its way to confirm that Garrett is still blind, but he still has his blind Prophet Eyes and his hair in his face, he has a new staff practically glued into his hand except for when he's dancing with Kayley (who would be guiding him at that point), and he still has some vague mannerisms that kind of indicate his blindness. He doesn't seem to react to any visual stimuli - there's no "I can see you!" reaction shot immediately after the magic healing Excalibursplosion, and no reaction shot to Kayley's entrance in her pretty white dress at their knighting. The only time he seems to look directly at anyone is, again, while he and Kayley are dancing, immediately preparatory to kissing her, which would obviously require turning his face toward her anyway. And Kayley's the one steering the horse when they ride off into the sunset.
Then does that mean the dragons were magically fused back together (ouch) because they were taken apart magically at the end of the film?
Devon hints that their being fused together in the first place was the result of inbreeding ("We're the reason cousins shouldn't marry"). At the end, they are magically separated, but instantly join themselves together again while the magic is still potent since they've learned to work as a team and have decided that they're better off as one.
This troper always assumed that the blindness wasn't healed because everyone else that was healed had been put that way to to a magical/unnatural incident (magic potion, Griffin attack) and that the dragons, being dragons, were influenced by magic anyway. The blindness was caused by natural means, meaning that the magic did nothing to undo it.
How does fusing with an axe enable a chicken to speak English?
ITS MAGIC SO I DONT HAVE TO EXPLAIN IT!
Having to communicate with it's commander?
Actually, DOES Garrett remain blind at the end? Before he was blinded, his eyes were brown. And after Ruber gets destroyed, Garrett's eyes do appear as a light brown color.
So what's with the Magic Leaves of Healing, the flying helicopter flowers, the thorny grabby hands, the burping lake, and all the other weird forest animated things that were never explained, commented upon, or so much as looked funny at?
I suppose they thought kids wouldn't see it as weird, considering the setting. Kids manage to believe in things like monsters, santa, the tooth fairy, and some odd things that differ from person to person (this troper was scared of trees because she thought they were alive). If kids can believe all that stuff in our universe, why not characters in a magical setting from a kids movie?
Ever notice how the Forbidden Forest is full of spirals? ...More seriously, though: it's a magical forest. Presumably the characters don't react all that much to the weirdness because they already know it's a dangerous magical forest - most likely that's why it's the Forbidden Forest in the first place.
What puzzles me is why this bothers so many people so much. Is it solely because it wasn't explained? Aside from A Wizard Did It, it seems to me that magic is part and parcel with the Arthurian canon—hello, Merlin is actually in the movie (not that he ever does anything magical)! So it doesn't seem surprising to me that the forest would have so many magical things in it. Presumably having lived there as long as he has, Garret is used to it all and so feels no need to comment on it. Kayley may either be aware the forest is magical (as the troper above pointed out, that's likely why it is forbidden), is aware of magic in general due to things like Excalibur and Merlin, or was told off-screen by Garret. Ruber being able to get a potion from witches falls under that too. (And it's not like witches are exactly unknown in British literature.)