What if the US annexed all of Mexico

You can expect some serious guerrilla. Even during occupation, Mexican rebels will continue to make the Americans' lives a nightmare, similar to Polish resistance during WWII, only worse. The Americans will have no option but to leave in less than five years, and the whole episode will leave whichever president is in power's reputation in shambles. Think of it as a Vietnam... right next to the US.

It would be a suicide for the US. It's ironic: Mexico never could defeat the Americans, yet they could've deliver a serious blow by getting annexed. If I had to guess, it would damage the US economy and leave lasting effects until the end of the century. Actually, this could have some serious ramifications. The US's economy is still limping and is unable to join WWI. The Allies are defeated and Germany invades France. Germany wins WWI.

I think we've just found a seriously cool butterfly effect here...

The South may view this with favorable eyes, seeing the fertile lands of Mexico as fit for crops and the indigenous people as slaves. You'll see many Southern politicians struggling to retain control of Mexico, establishing slave businesses and plantations. Many will try to gain local support. However, the Mexicans will view the Southerners with even more hateful eyes, attacking their plantations and murdering hundreds. There's the interesting possibility that the Southerners can cement their position in Mexico, perhaps even creating more slave states. This would accelerate the Civil War, with the Confederacy having a real chance of winning.

Think of it: The North is too economically-devastated to wage war, the South has immense resources and excess manpower. They're on the road to become a superpower.

Holly Molly, this is starting to get cool!

I can tell you as personal experience many Mexicans disregard who is their leader (I have some Mexican friends). The Southerners might simply "hire" a bunch of uneducated indigenous people and treat them as slaves. It's not like they were treated any better under the oppressive Mexican government of Santa Ana, one of the worst dictators in North American history. Also, keep in mind slavery was not illegal during the 1840's, so the US would have no legal say against it. As I said, this would certainly accelerate the outbreak of the Civil War, resulting in an almost assured Southern victory.

Okay, so this is the deal.

Educated Mexicans: Mostly central and eastern Mexico, would argue against slavery and demand better labor conditions.

Uneducated Mexicans: The indigenous, mostly south-eastern Mexico, would be exploited by Southerners.

The Southerners would basically run Mexico. They'll fill the country with plantations and mines, another lucrative source of income. The educated Mexicans will want reform, yet the Southerners would retain control of the country. There are simply not enough pro-North states in Mexico, and the Southerners are just too powerful.

De jure, I believe Northerners will do the job. But I think an increasing number of Southern troops will station themselves in Mexico, especially around the aforementioned lucrative regions. Once war starts, there'll be more than enough Southern troops already in Mexico. Insurgent militias and guerrillas will also be quite prominent. Slowly, the Northerns will loose much influence in Mexico.