The Xbox 360's best games are all First-Person Shooters or Sports. The 360 lacks good games from other genres. Sure you have xbox live arcade or viva pinata, but the 360 lacks games to support gamers who are fans of other genres like RPGs (FFXI is overrated and Blue Dragon hasn't come out stateside) and platformers. Xbox 360 is catering to hardcore gamers, and gamers older than 20. It does very well at serving its audience though with plenty of titles. Xbox Live is still the best online service even though it costs money. It is by far the most developed and advanced of the three.
The Nintendo Wii offers innovation. Nintendo's purpose was to expand the market by using simple controls, and revolutionary gameply to introduce gaming to young children, women, and seniors. Nintendo's main focus is actually to expand the market then to cater to its loyal fans. Nintendo Wii is similar to the DS in the aspect that it offers something new in gaming. When the DS was released, not many games efficiently used its hardware. It took years for developer to develop games that were unique. I believe the same holds for the Wii. In a year, we'll start seeing those games that use Wii's controls for unique gameplay experiences like Wii Sports, Wii Play and Wario Ware Smooth Moves. But the main audience for Wii will still be casual gamers and nintendo fans.
The Playstation 3 has many cons. It's hard for developers to make games for. It's overly priced when the difference in graphics is hardly noticeable. The PS3 hardly offers any games that justify its price right now. 2007 has great games(VF5, Heavenly Sword, MGS4) for the PS3, but not enough. Many of the games coming for PS3 will be ported to 360, like FFXIII. Blu-ray might just end up like Sony's betamax. Developers are noticing the lackluster userbase of the PS3. Why make a game for the PS3 when its cheaper to make the game for 360 which has almost 10x the userbase? Not to mention the tie ratio for software with PS3 is bad. For every 4 PS3s sold, 3 PS3 games are sold. If many owners of the PS3 aren't buying games, why make the game for PS3?
The PS2 had great games for all genres, so almost all demographics interested in video games picked it up. If there was a PS2 system this generation, it would in fact be the Nintendo DS. The Nintendo DS offers many unique games that cater to audiences, and with new developers(Square-Enix has announced 14 DS games from a year ago it was just 3) jumping on the wagon, it's still gaining momentum.
The sytem you decide to get should be based on the audience you're trying to entertain. Women, children, seniors, casual gamers, nintendo fans should get the Nintendo Wii. If you're a hardcore gamer and are easily impressed by graphics get a 360. If you want a cheap Blu-Ray player, George Foreman Grill, or Sony First Party Games, get a PS3. But almost all gamers should get a Nintendo DS because it offers games for everyone.