EDIT: UPDATED WITH A NEW UNLOCKABLES GUIDE. ONE THAT WORKS, I"VE TESTED THEM! 
I LOVE MARIO KART WII! I have had it since midnight and the wife and I have played sooo many races, lol.
That said, you will find that it doesn't start with many racers, courses, etc. They must be unlocked. Here is a copy/paste job from Gamespy for us here at X3F to enjoy all the unlockables! We have been at it all afternoon, so I can confirm that some of them work, but not others. If you find any corrections, post it for everyone to see!
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Unlockable: Mirror Class
To unlock the Mirror Class, win all of the available 150cc Cups in the game.
Unlockable: Bikes on 50cc Cup Races
To unlock the ability to drive bikes in 50cc cup races, earn all of the trophies on each 50cc cup race.
Unlockable: Victory Photos
To unlock both victory photos available in the game (which is basically a shot of your Mii with various characters), you have to fulfill two requirements. To unlock the first photo (which is of your Mii with Mario and Peach), earn a trophy in every cup for each class available. To unlock the second photo (which is of your Mii with each racer in the game), win each and every Grand Prix cup available in the game.
Unlockable: Medium Bikes
To unlock the medium bikes listed below, fulfill the requirements listed.
* Dolphin Dasher - Win the Mirror Star Cup
* Nitrocycle - Attain at least a one star ranking in all 100cc Wii Grand Prix cup races
* Rapide - Win the 100cc Lightning Cup
Unlockable: Small Karts
To unlock the small karts listed below, fulfill the requirements listed.
* Blue Falcon - Win the Mirror Lightning Cup
* Cheep Charger - Earn at least a one star ranking in all 50cc Retro Cup races
* Rally Romper - In Time Trial mode, unlock the expert staff ghost data record
Unlockable: Small Bikes
To unlock the small bikes listed below, fulfill the requirements listed.
* Bubble Bike - Win the Mirror Leaf Cup
* Magikruiser - Play Time Trial Mode on any eight courses
* Quacker - Win the 150cc Star Cup
Addition from mr caspartt
Unlockable: Karts on 100cc Cup Races
To unlock the ability to drive karts in 100cc cup races, earn all of the trophies on each 100cc cup race.
Unlockable: Medium Karts
To unlock the medium karts listed below, fulfill the requirements listed.
* Royal Racer - Win the 150cc Leaf Cup
* Turbo Blooper - Win the 50cc Leaf Cup
Unlockable: Cups
To unlock the cups listed below, fulfill the requirements listed. Keep in mind that the requirements are the same for each CC type on the circuits listed.
* Leaf Cup - Earn trophies in the Shell Cup and Banana Cup
* Lightning Cup - Earn a trophy in Leaf Cup
* Special Cup - Earn a trophy in Star Cup
* Star Cup - Earn trophies in the Mushroom Cup and Flower Cup
Unlockable: Alternate Title Screen
To unlock an alternate title screen that will be shown when the game is started up, win each and every cup available in the game.
Unlockable: Large Bikes
To unlock the large bikes listed below, fulfill the requirements listed.
* Phantom - Win the Mirror Cup
* Torpedo - Unlock twelve Expert Staff ghost data in Time Trial mode
* Twinkle Star - Win the 100cc Star Cup
Addition by mr caspartt
Unlockable: Large Karts
To unlock the large karts listed below, fulfill the requirements listed.
* Aero Glider - Earn at least a one star ranking in each 150cc Retro Grand Prix races
* Dragonetti - Win the 150cc Lightning Cup
* Piranha Prowler - Win the 50cc Special Cup
Addition from mr caspartt
Unlockable: Characters
To unlock the characters listed below, fulfill the requirements listed.
* Baby Daisy - Earn at least a one star ranking in each 50cc Wii Grand Prix cup races
* Baby Luigi - Unlock eight Expert Staff ghost data in Time Trial mode
* Birdo - Defeat 250 people in online racing, or play Time Trial mode on sixteen different courses
* Bowser Jr. - Earn at least a one star ranking in each 100cc Retro Grand Prix cup races
* Daisy - Win the 150cc Special Cup
* Diddy Kong - Win the 50cc Lightning Cup
* Dry Bones - Win the 100cc Leaf Cup
* Dry Bowser - Earn at least a one star ranking in each 150cc Wii Grand Prix cups
* Funky Kong - Unlock four Expert Staff ghost data in Time Trial mode
* King Boo - Win the 50cc Star Cup
* Mii (Outfit A) - To play as the Mii associated with your profile, win the 100cc Special Cup
* Mii (Outfit B) - To play as the Mii associated with your profile in a different outfit, play through and complete all thirty-two time trials
* Rosalina - Have a Super Mario Galaxy save on your Wii
* Toadette - Play Time Trial Mode on all thirty-two courses
Addition by FireBlade64
Hint: Avoiding POWs
Getting attacked with a POW can be a bummer, but if you flick upward your steering wheel or Wii remote, you can avoid it. You will spin around, and you will lose your currently-equipped item, but you won't slow down and will avoid the devastating result of being struck with a POW.