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: Mario Tennis
Crowning Music of Awesome: The GBC version of Mario Tennis has some surprisingly elaborate music for a Game Boy Colour game. Especially great when you get to the Semifinals of the Island Open.
One could argue that the awesome music even starts in the Preliminaries
Fan Nickname: Yoshi Tennis Open to the 3DS game because of the overabundance of many Yoshi colors as singular characters.
Relationship Writing Fumble: Elroy and Tori are supposed to just be doubles partners, but behave and interact like a couple. At a few points you almost expect them to smooch. The GBA version also has this with whoever your doubles partner is, but a milder version. Another notable mention is Dweezil and Mynx.
Scrappy Mechanic: The primary way to earn coins in Open is to play the Special Games... and that's it, unless the player wants to replay the tutorial for 200 coins each when it takes 155,000 to purchase everything in the shop.
The power shots are this for players of the game who actually play tennis in real life. Mainly because if you can pull off a good one it's an instant win for whoever launches it (player or NPC).
The charge shot in the N64 version. If your timing is off, you'll accidentally activate it (you have to hit the button slightly BEFORE the ball gets to you), and then you're stuck moving at the speed of molasses, which can very easily give your opponent(s) the point. There's a way to cancel it (by pressing L) but by the time you realize it's happening, it's probably too late to react that fast, ESPECIALLY in the later cups. (Not to mention that the L button is hard enough to access with the way you need to hold the N64 controller anyway.)
That One Sidequest: The Reaction Training Minigame in Mario Tennis: Power Tour. It is a Rock-Paper-Scissors game against a robot where you have to win x number of times before a timer hits zero. The second level alone is tough, but the third level requires godly reaction speed and massive amounts off guesswork and pure luck. Reaction is needed to get some of the more powerful and useful Power Shots. Most likely, you will be stuck doing Level 1 over and over again, tediously grinding until it hits level 10.
Galaxy Rally in open. It's a constant volley where you have to return the ball to a Luma while avoiding the holes the ball leaves when it hits one of the four sections of the court. As the game progresses, there will be more holes, so you have to aim at the only parts of the court where there's still solid ground. Worst part? You need to clear the level 3 version of the minigame (volley 100 shots) to unlock Luma.