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.
Awesome Music: Dark Lord/Emperor's battle theme from Streetpass Quest 2.
Character Tiers: In Mii Force, Brown, White, and Light Blue shirts are better than other shirts. Brown and White can destroy purple plasma shots while Brown also gives a perpetual shield against physical attacks. The only drawbacks of Brown is that when it blocks a physical attack it launches you in the opposite direction, potentially into another trap and Brown shirts can be really hard to find. Light Blue shirts just have good range, rapid fire, damage, and destroy fire based attacks. While other shirts like Blue, Red, and Orange are good in general and other shirt colors are good in certain levels (2-3 for Black, 3-1, 4-1 and 4-3 for Light Green), Light Blue, White, and Brown are the best shirts to have on your ship during most levels of Mii Force. They make arcade mode a bit easier. The only useless power is Dark Green. See The Scrappy below for details why.
In Find Mii, Yellow, Light Blue, Dark Blue, Red, Black, and White shirts are often required for passing certain levels. In addition, Red and Dark Blue magic is handy for armored enemies where, if not strong enough, can still deal damage that way, and Light Blue and Light Green magic can be handy against the Green Slime.
Demonic Spiders: Any enemy that takes very little damage from your attacks, or uses any type of magic (e.g Fortifying Ma-gic, Mirage Magic, Level Down Gas, Nullifying Magic) in Streetpass Quest 2 / Find Mii 2.
Mummies are especially infuriating since they usually cost you a hero before you even do anything.
Certain Reaper enemies will spew Cursed Gas, which often prevents your heroes from acting at all. A Dispelling Draft might as well be mandatory.
Rescued from the Scrappy Heap: Brown Shirts weren't that useful in Find Mii, as they could only summon another hero, and it was always a random color. But in Mii Force, they have a drill weapon, handy for defense.
You know Dark Green Shirts? In Find Mii 2, the combined magic of two of them instantly maxes the level of the next warrior.
The Scrappy: Dark Green Miis, whose abilities are practically useless in both Find Mii and Mii Force. In Find Mii they double your next warrior's level. It sounds nice but unless you have a bunch of level 1's and the Dark Green is in front then its worthless. In Mii Force Dark Green gives a bouncing ball which does bad damage unless bounced off an object, bad attack radius, and offers no defense like Brown or White shirts do.
In Warrior's Way, people who have no street passes, thus their army consists of a single useless soldier.
Scrappy Mechanic: Some of the potions you order may turn out to be defective, wasting your hero and your Play Coins. Thankfully removed in the update that included the additional games, considering they're by far the most expensive thing that consumes Play Coins in any of the games outside of hiring additional troops in Warrior's Way.
In Puzzle Swap, using Play Coins to obtain pieces is not a surefire solution to a dearth of Street Passers - the game can (and often will) give you pieces you already have, sometimes several in a row. This can become extremely aggravating when one repeatedly gets dud pieces on a panel with only one piece missing.
Stop Helping Me!: Mr. Mendel quickly falls into this. After playing the game for a while, many players will be shouting at Mr. Mendel to just shut up after repeating the same piece of advice countless numbers of times and just wanting to get to greeting the guests.
It's even worse after one has grown all 80 plant breeds, because he'll still offer advice on how to grow new breeds.
That One Achievement: Mii Force's level 4-3 third gem is notoriously hard to obtain; during the descending sequence, both of the huge submarines must be destroyed at the same time. It's said the best way to pull this off is to use three level 3 Light Green turrets; colors that are very hard to come by.
Warrior's Way and its "battle 300 visiting monarchs" plaza ticket. Depending on where you live, this can be almost impossible to achieve. And even in an area with lots of 3DS users, it can still take a very long time.
Also, 3-3's fourth gem in Mii Force; you have to destroy both of the green Ferris wheels at the same time. Try doing this when there are a ton of projectiles flying at you.
That One Boss: Like stated in the main article, Emperor Fynale in Warrior's Way is this: he has 8 times as many troops as you do you when you first face him, you can't spy on him to find out what kinds of troops he'll use and the match goes on for 5 rounds instead of the usual 3.
The worst room of this kind in Find Mii/StreetPass Quest 2 not only has an enemy with a shadowlight shield in front of it, but it's also a green slime, meaning that you can't bruteforce your way through it with magic.
Rooms covered in darkness can ONLY be passed with White magic, forcing all other heroes in front of the White hero to flee immediately upon entry. Unlike shield rooms, you can't re-order your heroes to put White in front. And no matter how many heroes you hire, if none of them are white, every one of them runs away and no progress is made. At least you never have to light the room again on that playthrough once you light it once.
Streetpass Quest 2 / Find Mii 2 has a fire room where 2 consecutive Dark Blue heroes are needed to put out the flames, a poison room that requires 2 consecutive Yellow heroes to send away the poison, and an ice room that needs 2 consecutive Red heroes to melt the ice. It also introduces the Shadowlight Shield, which can only be destroyed by a Black/White team. On the bright side, Find Mii 2 lets you bring back old heroes in the same way Wandering Heroes are hired, so having some old Streetpassed Miis could provide you with the colors you need.
The Sealed Passage in Find Mii 2's Secret Quest. It contains a powerful Mummy enemy and the Reaper General. The Mummy will scare away your Miis while the Reaper spews curse gas and has a defense of 2. And although you might think it would be best to get rid of the weaker Mummy first to save your Miis, if you do, then the Reaper will REVIVE it with full HP!
The roller coaster level in Mii Force makes the game into a literal rail shooter, only letting you move vertically and there are two forks in the path. Especially annoying in that you must take the hard routes in order to get some of the treasures.