There's not a lot to say about Tennis, but that doesn't mean it's a bad game. It was one of the most playable sports titles on the NES, and despite some fancier, more "realistic" tennis games released later in the system's life, it did a fine job of holding its favor with players until the SNES' Super Tennis came along and did very much the same thing with better audiovisuals.
Activision's Alan Miller pioneered true (beyond Pong) videogame tennis on the Atari 2600, making the game playable in pseudo-3-D by adding a shadow to the ball, allowing the player to judge height and position in relation to the court. Nintendo's take on the game retains this critical visual cue, augmenting it with ball sprites of different sizes to communicate distance and altitude as well. The controls are tuned to be forgiving to a degree -- similar to Wii Sports' Tennis event, we don't have to have pixel-perfect placement and timing to return the ball, but we have to be pretty close; when we miss a return, we know we were standing in the wrong spot and/or swinging at the wrong time.
While there's no real swing control beyond timing, all of the standard tennis strategies are effectively supported -- we can slam the ball just over the net, try to send it just within bounds from a more distant position, or send it high into the air to make it more difficult for our opponent to judge its location.
I've played the NES version a fair amount over the years, but my favorite memories of Nintendo's Tennis actually come from its VS. system coin-op incarnation, where two players could actually play against each other. The NES version allows a singles or doubles game, but the computer always occupies the upper court, so humans can only play cooperatively. This makes sense, as the game would be rather hard to play from the upper-court perspective; it's much easier to judge the ball when it's coming toward the player. But I miss the one-on-one competition the VS. system allowed, with a separate display for each player.
And of course, Mario himself takes on the first of many cameo roles, playing referee for the match. The best part is that we can step outside the court far enough to almost (but not quite) take a swing at him when we're unhappy with his calls, making for memorably hilarious freeze-frame moments:
Nintendo's Tennis was an early title that still holds up pretty well -- it has been superseded by more realistic and/or active games on modern consoles, but there's a lot to be said for well-tuned simplicity.