Re: flight-- Flight impacts the game in two main ways --
- Eliminating challenges that can be solved simply by being avoided (melee only enemies in the way but not guarding something important, a canyon).
- Eliminating challenges caused by being unable to access a location (the lever to open the dungeon gate being across a neigh-bottomless pit or sea of fire, etc.).
The former is only an issue if the flier's ability can be applied to all relevant people (they can carry the whole party, etc.), and for the relevant period of time. This is why flying carpets (whole party, always on) are rarer magic items than winged boots (one person, 4 hour max). The later issue can be solved by a single flier with limited flight, but is also the one with any number of other solutions all the way from level 1 (mage hand, summon critter spells, etc.). It's also something of a diminishingly common situation, as the game moves away from dungeons that only make sense as challenges to treasure-hunters.
Re: unserious games -- whether Faeries, Kender/Tinker Gnomes, or Dark Sun post-apoc aesthetic are more/less serious than the other is always going to be a matter of perspective. Just like whether Ravenloft is D&D with a scariness upgrade or D&D wearing Halloween costumes. The default game treats
elves,
dwarves, and
goblins as serious creatures, there's no specific reason that faeries can't be as well (
faeries can wear
combat boots, after all). Dark Sun likewise is either brutal serious survivalism (hampered a little by no edition having great rules for such) or I guess I'll call in Mad Max posturing* again exclusively based on how one internally frames them. Kender/Tinker Gnomes I think are inherently silly, but whether designated comic relief makes the whole of a thing less serious or not, I think, is where the matter of taste aspect comes into play.
*I consider the term 'edgelord' to be itself nonsensical. Tantamount to that childhood insult 'they think they are so spe-shul!'
Exactly how serious the game is supposed to be is likewise always up for grabs. EGG hiding dad jokes in the spell components communicating "only as serious as you want it to be".