Lethal Joke Character: You can set the Mario characters onto any difficulty in the GBC game, but all of the story characters, regardless of their class, will automatically be set on hard each time you play an exhibition game.
Character Customization: A new feature in Open allows players to dress their Mii up with various gear, changing their stats to the player's taste.
Climax Boss/Final Boss: A. Coz (in Singles), and The Costello Brothers (in Doubles) in the GBC version. Willy Costello, joined by Sheri in Doubles, in the GBA version. Winning nets you the trophy And an invitation to the Mushroom Kingdom for a chance to face off against Mario.
Color-Coded for Your Convenience: In Mario Tennis Open, in addition to the usable-by-default green Yoshi that specializes in speedy play, QR codes unlock more Yoshis with different stats, such as yellow specializing in power, red specializing in technique, white being tricky, and black being a Jack-of-All-Stats.
Confusion Fu: Tricky/Tactical characters like Boo in the N64 and GCN versions, Bowser Jr. in the GCN version, and Willy Costello in the GBA version. They have insane amounts of spin on their balls which can catch a player off guard.
Strangely, in the GBC game, a speed-oriented character, Spike, has a tricky-type move; his drop shot (a deliberately weak shot that barely makes it over the net) curves to the right, making it harder to follow. Meanwhile, A.Coz has a very powerful lob (high) shot that will fly high, regardless of what part of the court he's on.
It's worth noting that while you can get your serves to curve, you and the computer can do this. Spike however does this with his drop shots, something you can never do.
Cute Clumsy Girl: Daisy's tournament win animation has her tripping after getting her trophy.
Defeat Means Playable: Every tennis opponent that you play against in both handheld versions. This includes Mario himself in the GBC version. For the GBA version, Mario Series characters are instead unlocked at the start and you get Star versions of them instead.
Epic Battle Boredom: Victory Medals in Mario Tennis Open. They're glorified versions of dog tags and you can collect them all as long as you defeat a different opponent online. Be aware they can do the same to you.
Fragile Speedster: Yoshi, Toad in the N64 version, Baby Mario in the N64 and GBC versions, A. Coz in the GBC version. Though since there's obviously no sense of "damage" in tennis, it's more like "Speedster who can't hit the ball very hard."
Jack-of-All-Stats: Mario, alongside other All-Around characters such as Luigi. Also Willy's partner Sheri and Rank 1 Varsity Player Elroy in the GBA version. In Open, out of the unlockable-via-QR Code and differently colored Yoshis, the black one is one of these.
They do have their differences though. Luigi for example had notably better reach in the N64 version, while Mario was more powerful.
Jerkass: The Costello Brothers, A. Coz, and B. Coz, are minor examples. Tori in the GBA version plays this a bit more straight, while Dweezil is a somwhat more milder example. Averted with the third Costello brother, Willy, who just desires to play against a worthy opponent regardless of the outcome, as well as his partner Sheri who is just confident in both her and Willy's skills, Willy even emphasizes that he's thrilled to be playing in the final regardless of the outcome.
Insufferable Genius: Tori. Elroy and Dweezil are mild examples. Emi at first appears to be a mild version of this but turns out to be a real sweetheart. Mason thinks of himself as this, but is more of a Small Name, Big Ego.
Loads And Loads of Yoshis: Mario Power Tennis gave Yoshi an ability to change colors whenever he used his defensive power. This was deconstructed in Mario Tennis Open, where his colors are now QR singular characters for different categories.
Lightning Bruiser: By spending your points wisely, you will be able to make your player character into one of these.
Mighty Glacier: Heavy characters like Bowser and Petey. For a human example, look at Wario or B. Coz. An example of the former in Open, Dry Bowser, takes it a little far with the glacier part — his walking (as opposed to running like everyone else with feet) movement is the absolute slowest in the game, as if his bones were joined together by molasses joints, instead focusing on his power.
Mythology Gag: Metal Mario in Open obviously references the Metal Cap powerup in Super Mario 64, but his topspin shots and Fire Flower shots reference the Fire Flower — his shots for either of those stay low to the ground and bounce repeatedly, and the "flames" around the ball make it look like a fireball from the powerup in question.
Original Generation: Present in both the GBC and GBA games. They serve many roles such as playable Story Mode characters, helpful advisors, opponents, and more. Alex of the GBC game becomes head coach in the GBA game, and Harry, Kate, and Nina become very high ranking coaches.
Power Glows: Rackets glow whenever Power Moves are available.
Red Oni, Blue Oni: You (blue) and your partner (red), also Elroy and Tori (top varsity opponents). Inverted with the differently colored Yoshis in Open (unlocked via QR code on the game's Web site) — Red and Pink have more focus on technical skills while Blue and Light Blue focus more on speed like the standard green one.
Save Scumming: That CPU in Star Tourney is at 40 while you're at 0? No problem! Save to keep your record intact!
Secret Character: DK Jr. and Shy Guy in the N64 version. The handheld version had every tennis opponent you faced during either singles or doubles (meaning you had to play both singles and doubles to unlock them all) and the GBC version included prominent Mario Series characters that were unlocked by beating them in the last tournament (Mario and Peach) or by connecting your game with the N64 version (Yoshi, Waluigi, Wario, and Bowser).
Connecting the GBC version of Mario Tennis to the N64 version also adds four human characters to the N64 version: the characters that you go through the Story Mode and their doubles partners. As there is no current way of replacing the Transfer Pack method, and no way of getting or connecting to the GBC game currently, you cannot unlock those characters in the VC version. Partially justified in that these characters stats depended on how you had leveled them up in the GBC versions.
Spotlight-Stealing Squad: Wario and Waluigi in the Mario Power Tennis opening movie. Since Waluigi debuted in Mario Tennis 64, it's not that surprising.
Stone Wall: Waluigi in the GCN and GBA versions, also Wiggler. Last year champ Dweezil in the GBA version.
Sugar and Ice Personality: Dweezil the champ of last year's Island Open in the GBA version. He refuses to speak with you on the grounds that you are a newcomer in the tournament and thus "fresh meat", but warms up to you before the start of your next tennis match against him.
True Final Boss: Mario serves as this for both Singles and Doubles play (joined by Peach) in GBC. He was degraded to just Singles play Final Boss in the GBA version where Bowser and Waluigi seized the Doubles spot.
The Unfought: Kevin, the Varsity Champion of the GBC game (and Varsity Coach in the GBA version), is never in a tennis match with the player (he is defeated in the Island Open by the Costello Brothers in both Singles and Doubles), and thus is never a playable character, despite Mark and Emily, the Junior and Senior Coaches respectively, being playable. In the GBA version, none of the original playable characters that appear in cameos are unlockable, save for obviously the Mario series characters.
Other characters who're seen in the Island Open (Ellis, Edgar, Frank) are never played. Interesting that Edgar and Frank are just palette swapped.
Tsundere: Tori starts off as an Insufferable Genius but when you visit her in the infirmary after she sprains her ankle, she's considerably nicer and wishes you luck in the final.
Wake-Up Call Boss: In the GBA game when you play in the Mushroom Kingdom Tournament for the first time, the first opponents in both the Singles Tournament (Peach) and the Doubles Tournament (Luigi and Donkey Kong) are considerably harder than the later opponents up to and including the Final Boss of each.
Weak, but Skilled: Peach, Daisy, Waluigi in the N64 and GBC versions. Tori in the GBA version is also an example.