These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
YMMV: The Legend of Zelda: Four Swords Adventures
Demonic Spider: Any time Shadow Link manifests in the air. He's invincible like this and will repeatedly drop Giant Bombs on you; all you can do is avoid them until you escape the area.
Fanon: Dark Link spawned a ton of it; chances are if it's not of Ocarina of Time's Dark Link, it'll be of this one. The manga definitely helped this along.
On the "They Changed it" side... it's a multiplayer Zelda game, so that should say everything. Yes, there is a compelling single player mode... in which you play four Links at the same time and it has no overworld in the classic sense of the franchise.
Those who complain about "It's The Same" argue that they recycled too many graphic assets from previous games, even if they were enhanced. Plus, some argue that it plays too similar to the GBA version (despite that FSA blows it out of the water in every possible sense), if not to A Link to the Past itself, since it uses many items from that game.
If a GBA is disconnected, the game freezes for a while before returning to the title screen if the controller is not reconnected. This only allows enough time for the event of the cable accidentally being unplugged. If the batteries in your GBA died you better hope you saved recently. Worse yet, the design of the GBA SP and the GBA-GCN cable combined means you can't have an SP plugged in with an AC adapter at the same time you have it connected to the GCN.)
The fact that you needed to buy a GBA and a connecter cable to begin with is bad enough. When the game was released, that would be almost 200 bucks down the drain if you didn't already have a GBA, and nowadays it's maybe $50-80 on the used games market, but almost impossible to find.