So, read through all these responses, looked at all the arguments... I've been wanting to add my thoughts/commentary to this for a while now, and I've been kind of pondering it ever since the news was announced on Facebook. I don't know if anybody really cares that much about my opinion, but if so, then here are my thoughts on it.
It's a show for preschoolers. I will admit, this is somewhat disappointing to me. I don't have a ton of patience for TV shows that are really childish - that would really be something that would turn me away from the show. I know they've done this for other classic movies, such as Jake and the Neverland Pirates for Peter Pan... but tbh, I saw one episode of JatNP and was not impressed by it, not to mention that, from what I've heard, Captain Hook just is no longer menacing as a villain in the show. Kind of a shame. I just hope people realize that it's different from the movie, and it's not really worth comparing them all the time.
Regarding fan age: a lot of the people I've seen on FFn are around 13, mostly girls, a lot of them TLKII/KovuxKiara fans... which is as it was when I joined. I was also 13 when I started on that site and became a TLK fan, which was around the time when the movie was being re-released in 3D and the fanbase seemed to be experiencing a bit of a boom. Now I'm 16, but assuming that I'm the youngest member in the community is doubtful. Most people seem to be around my age, maybe a year or two older, at least amongst the serious fans. There are still quite a few 13 year-olds that post a couple fics on FFn and some that come on here. There are people who are even younger here, as well.
In any case, I hope they at least make some effort to appeal to older audiences. If the show is exclusively geared towards young children, then I probably won't watch it all that much. As it is, I'm 'cautiously optimistic', as it has been said. I'm
hoping it'll be good, perhaps have an MLP:FiM thing going on if we're really lucky, but again... fingers crossed. I'll probably catch the TV 'movie' premiere and peruse a couple episodes online or on Netflix to see if I like it. I'll be in college by then, so... not sure if I'll have a lot of free time or not.
Canonicity issues? Don't think that'll be a problem. As Julie said, I think it's safe to assume that Disney isn't going to totally mess up everything. Maybe a few episodes might get a little bit out there, as the T&P series seemed to do, but provided that you suspend disbelief a little bit, I don't think there will be anything really contradictory continuity-wise. Kiara is still the queen/older sister, and I'm assuming everyone else will just be about a year older than they were at the end of TLKII. That's a reasonable decision. And I don't know why everyone assumes they're bringing Mufasa back from the dead, or making Kiara a cub, or anything like that. We have no evidence to support such a claim, and aside from that, it'd just be ridiculous. Disney knows better than that, I think. Mufasa will probably show up via flashback, as he did in Kopa's stories.
Speaking of which, as Julie also said, Kopa is not a canon character. He exists only in the book universe. If they added in Kopa, then people would probably wonder why Chumvi, and Tojo, and Tama, and Kula, and all
those cubs aren't in the TV show, too. Thing is, they have no reason to really add in Kopa at this point. Although I feel like it may quiet the people who always complain about how Kopa wasn't in TLKII for a little while, it just doesn't make sense for the cub to be Kopa.
I will admit, though, I'm a bit surprised that they're giving Simba another son. Personally, I'm okay with that, it could be interesting. I'm just worried about the fan reactions that's going to get... (OMFG they KILELED
my KOPAAAAAAA!!! EHRRRRRRMERGARRWD!! DISNEY, Y U NO HAVE KOPA?!?!

)
Ahem. No offense to reasonable Kopa fans, but I'm sick of that "Fluffy was Kopa!!", "Kiara replaced Kopa!", "Kiara shouldn't be in the movie!" BS. Hopefully this doesn't prolong that drama. *rant over*
I do have a few questions as to the characters, though. I heard that "Kion" was Swahili for "young warrior" or something like that? Maybe? I dunno. Either way, I don't mind his name, just like I don't mind that he's Simba's son - it's head-scratch-worthy, and kind of a surprising move from Disney, but I don't think it will hurt. Overall, I like the way he's designed, especially since he has a bit of a different (read: Scar-like?

) mane shape.
The pawprints are a bit of a 'what?', though. It seems... I dunno, Mary-Sueish? Forgive my Fanfictionite jargon, but I've seen plenty of things like that in bad FFn stories. Overall, it just reads like something a 12 year-old girl came up with without realizing how silly it sounded.

If it's a mark Rafiki gave him, that's one thing, but in any case I'd like some kind of
decent, non-cheesy explanation. If it turns out it's 'special' or has 'magic powers', then that's big eyeroll points towards this series, at least in my view. It seems like others agree, so I guess I'll spare further explanation aside from saying that I really,
really hope this series doesn't go down a path like that.
I'm also wondering, as others have already, about the other characters specifically absent in the announcement. I mean, I'm wondering why Kovu and Kiara don't have cubs at this point, or why Kovu doesn't appear to be present (again, an explanation for this might be in order - if Simba and Nala had enough time to have another cub, then why not Kovu and Kiara? Did they just want to wait or something?). Granted, we don't seem to know a lot about this series yet, so Kovu could very well be in it... and personally I hope that's the case, even though I'm not a
huge fan of him like some people. Removing him would cause omgmasshysteria, unless there was a very good reason he wasn't there (in which case it would still probably cause omgmasshysteria).
The fangirl in me is also wondering if Scar will make an appearance, since Mufasa (?) seems to be slated to appear. I dunno, I think it'd be a little unfair if Mufasa was there but Scar wasn't, and hopefully if there's flashbacks we can see a bit of a different side of him, perhaps at a different age (oooh, like maybe we can see Scar as a *teenager* (!) with Mufasa, and they still maybe got along.

*fangirl squee* Anyways, sorry). Overall, I think that would be worth exploring... but since it's a kids' show I'm frankly a bit worried that they will cut corners and go the 'Scar = bad, Mufasa = good' route, since a lot of times characterization/character development isn't a priority in these shows most of the time. *sigh*
Anyways, the animation appears to be at least decent. Not the same as the movie, but then it would be completely unreasonable to expect a TV show to have that quality of artwork anyway. The thought that the TV 'movie' will have better animation and the rest will be TV-grade is reasonable and reassuring. Some effort will be put into it, at least. I would really like to know who is writing/directing, though.
I have to say, the thing that I like most about this, overall, is the effect it'll have on the fanbase. Kion fanart has already been appearing, and I'm imagining that plenty of fanfics will follow to 'catch up' with these new developments as authors 'update' their TLKII/post-TLKII fics (as these comprise a huge portion of TLK-related stories out there). Which, again, I am totally okay with, regardless of whether the fics are good or not. If it gets people writing, I don't really care. Hopefully it'll bring new, fresh blood into the fandom. Who knows, maybe one of them will end up being a good fanfic author/artist when they're older because TLG brought them to explore the movies and the Internet TLK community.

Either way, I will benefit from this, personally. I'm honestly just glad they're adding new material, since that means that they aren't 'done' adding official material yet and this fandom does indeed have a future. So who knows if they'll stop there? Even if we don't like The Lion Guard, that's not to say better things won't come after that. Worst case scenario, the show sucks, I read a bit about the general plot and characters of the series, then take some general ideas/themes/events from it and make a fanfic based off of it... only geared towards us older fans. Boom. Respect for TLG restored, my fans are happy, some of the burn of destroyed expectations is soothed. Best case scenario, though, the show's a great kids' show with appeal towards older fans, it blows the other Disney Junior shows out of the water, there's a revived interest in the fandom, and plenty of new fans show up to boot. Oh, and don't forget about extra movie, soundtrack, and merchandise sales.
Overall, waiting to see how this pans out, though at the very least I don't think it will be a big negative for Disney. It may even be a big plus. But that's just my opinion. Again, fingers crossed!