This feature refers mostly to the visual style of a game. The essential idea here is designing cartoonish characters with small bodies and big heads just like a baby would look like. This feature can be seen in a lot of popular games such as the new DS Zelda games (also de Wind Waker), LittleBigPlanet, MySims, Castle Crashers, and the next coming title Mini Ninjas.
This style is appealing basically because it gives fun to the whole visual scheme of the game: the characters funnier, their movement clumsy, the lights clearer and the colors vivid. These traits describe what families, casual gamers and probably your girlfriends would like to see in a game.
However this baby head feature succeeds even applying different proportions to characters. It works well as long as the visual style ends up being cartoonish, funny and with clumsy animations. Baby heads just make the style go across the safe road.
This is a very important feature that should be included in almost every game, even if the game is not intended to be popular. Multiplayer modes serves for multiple good purposes if designed well. For instance, multiplayer can ensure replayability in a game by extending the game experience to short (sometimes long like in Civilization games) competitions with friends. It also allows to the creation of bigger communities faithful to the game and if implemented single mode as well as multiplayer mode it will also attack a larger audience.
Probably the best example for a good multiplayer mode that extends the game to a richer experience is Castle Crashers. Castle Crashers has a story mode which can be played cooperative (both ways is fun to play) and also expands the game's life time by introducing some competition modes.
***DISCLAIMER ALERT!: The following example is based upon a game I have worked on. If you will feel offended in any way about this, please skip the next three paragraphs. You will miss the opportunity to learn about the things I work on, but I can forgive you! :P ***
Decisions in game design should be done trying to make the make the game as replayable as it can. There are a lot of reasons why. People will most likely buy the game if they know they will be able to play the game for much longer. But although I mentioned implementing a Multiplayer Mode is the best way to do this it is not THE WAY. As an example I will mention Cellfactor: Psychokinetic Wars, a FPS game which main experience is based on its multiplayer mode (it also has a single mode which consists of defeating challenges).
Cellfactor is a fast paced, well balanced game and its very fun to play. It allows the common FPS modes and implements some creative abilities to the player which make it stand good as an arcade game. However this game is too focused in a very specific gameplay (being this the mainstream FPS gameplay). Also, the multiplayer mode in Cellfactor can't represent an extension to the game experience as the multiplayer mode is what the game is mostly all about.
Being this said, it's clear that although Cellfactor has a great implementation of a multiplayer mode it doesn't add up as a bonus feature and for this reason it is a game that has to find support on its core game elements. [This game is still popular in XBLA so I heavily recommend it]
Finally, customization in a game is the one feature which ensures the player's personality to be projected somewhere into the game. This also fits well in conjunction with creation of online communities because of the custom profiles that some games allow.
Good examples of customizations in games can be found almost everywhere since this has become almost like a general rule to good game design. LittleBigPlanet, The Sims, and RPG games such as Fallout 3 are some of the games that uses customization.
Customization doesn't limit itself to aesthetical features in characters and profiles, but can also enhance some gameplay by allowing the player to customize his own power ups, skills, perks which can also be gathered within the gameplay. And although this type of customization is commonly used in RPG games, it has also been used in other genres such as First Person Shooters (like CoD, Cellfactor), Simulation Games (The Sims games), and others.