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: Sonic Heroes
Anti-Climax Boss: Metal Overlord. While Metal Madness is mildly challenging due to having one instant-kill attack, Overlord is one of the easiest bosses in the game!
Best Level Ever: Many players hold Egg Fleet as the favorite, not only for the sheer sense of vertigo of being high up in the air (even compared to other airship levels in the series), but for the challenge offered by having to constantly dodge gunfire from the ships, and the sheer amount of alternate paths available.
To put the cherry on top, the level features the playable trio creating a chain reaction that destroys an entire battleship with Rocket Accel. And Team Sonic's version allows you to do it three times, with the third time standing in for the Goal Ring. Quite expectedly, there are a number of fans who are disappointed that Seaside Hill, essentially the Green Hill Zone throwback of the game, was picked to represent Heroes in Sonic Generations over Egg Fleet when theGreen Hill Zone was already in the game.
Character Rerailment: The Sonic Adventure series turned Knuckles into a dead-serious guardian, in sharp contrast to his Sonic 3 & Knuckles portrayal of laughing at Sonic at every turn. This game was able to reconcile the two portrayals into a deceptively stoic Lancer who gets smug at getting one over Sonic.
Metal Sonic copies Chaos's data from Chocola and Froggy. Chao are very closely connected with Chaos, and Froggy was once possessed by Chaos. When Metal Sonic enters his One-Winged Angel transformation, he does so by merging with one of Eggman's battleships. This is because he copied Chaos's shapeshifting powers. However, while Chaos is made of water, and thus merged with Station Square's reservoirs, Metal Sonic is made of metal, so he used a battleship to gain mass.
Metal Sonic's One-Winged Angel form is ridiculously easy because his ego is three times larger than his size.
Likewise, in a game running on the message of the power of teamwork, it makes sense that, once the four trios realized they really had no need to butt heads and united against a common foe, Metal Sonic's elaborate ploy was so quick to fail after he revealed himself to be the game's true mastermind.
Goddamned Bats: In the Hang Castle and Mystic Mansion stages, the ghosts that pop up everywhere.
Egg Bishops/Egg Sorcerers, one enemy with two different forms distinguishable by color. Egg Bishops (orange) have the ability to heal themselves and any enemy nearby. Egg Sorcerers (purple) can cast a spell that steals all your rings. And the worst part is if you use a tornado attack on them, it just flips them over to the opposite form, making a bad situation potentially even worse.
Harsher in Hindsight: In this game, Metal Sonic ends up backstabbing Eggman and trying to defeat Sonic himself by gaining more abilities with implied modifications. Three years later out-of-universe, Metal Sonic does it again in Super Mario Bros. Z, only it ends up being far worse than before, where he (presumably) murdered Eggman while backstabbing him, decimated a large percentage of the planet's ecosystem, and also murdered all of Sonic and Shadow's friends.
That doesn't even cover that annoying "WHEEE!" noise that Tails makes when you boost through the air. Still, you have to give the kid a break; the boy who voiced Tails (taking over from the VAs in the Adventure series, both of whom had hit puberty) was about 11 or 12 at the time.
"MUST BE MY LUCKY DAY!!!"
"Take this! Take this! Take this! Take this! Take this! Take this! CHARGE!!!"
Anyone who is the point man when you go into the cannon in Bullet Station. The team is shot out of the cannon and can pick up item balloons as they go through the air, subjecting the player to the snippet they say over and over.
The level system was considered highly annoying to many as well.
When hit by an enemy, a character gets stunned for a second, giving you less time than usual to collect your rings before getting hit again.
Enemies having life bars were a constant early complaint, since they messed up the flow of the game (especially if your characters weren't leveled, which meant that only Power-type characters could reduce their HP without additional effort, and later enemies carried HUGE life bars AND barred progress until defeat), and turned parts of the game into a crude Beat 'em Up instead of a traditional speed-based puzzle/platformer.
Flying formation can also be a pain, as you're forced to carry your teammates and speed is reduced greatly is this formation. And if your fly meter depletes fully, you sink like a rock, which can be frustrating if there's a Bottomless Pit below you, which is to say, quite often in fly specific areas. Attacking in flight formation can also be rather tricky, as it can be hard to aim.
The Special Stages, too, especially where you have to catch the flying emerald. Doubly so as Team Rose, which is a little bit slower than the other teams.
This is especially jarring as there's a sudden and unexpected Difficulty Spike between the 2nd and 3rd Emeralds. The Yellow Emerald somehow manages to be harder to obtain than all other Chaos Emeralds in the game barring the final one.
Rail Canyon and its following level, Bullet Station, for consisting of little more but rail grinding and virtually zero safe ground below you.
Tough Act to Follow: Since this game followed the immensely well-loved Sonic Adventure 2 (both the original Dreamcast version and its Gamecube Battle port), Heroes did, and still does to this day, get a lot of bile from fans for really slippery controls and its utter repetitive structure.
Visual Effects of Awesome: Metal Sonic's One-Winged Angel, which involves a fast-paced sequence of parts of a blown apart tower assembling together into his new Metal Madness form, with Metal's head morphing into some... thing that completes his transformation, all lasting for circa 50 seconds.