In what situations would somebody be charged with felony DUI as opposed to a misdemeanor?
Whether it is charged as a felony or a misdemeanor a DUI is always serious - driving under the influence of drugs or alcohol not only puts your life at risk, but also the lives of anybody unlucky enough to be driving near you when you choose to drink and drive. This is one of the reasons that DUI laws are so strict - because you can cause great injury (or death) to innocent people.
Felony vs. Misdemeanor
It is pretty uncommon for a DUI to be charged as a felony if you are a first-time offender unless you have cause extensive damage to property or injured another person. If you have been charged with a felony, however, you should immediately look for a qualified attorney who specializes in DUI's to represent you.
Even though you have been charged with a felony you may be able to reach a plea deal where you will be charged with a misdemeanor - again, I can't stress enough, you need a good attorney if you are facing a felony DUI.
Of course you don't want a criminal record, but having a felony criminal record can limit your ability to find a job, secure credit, travel outside the country, etc. as opposed to a misdemeanor which will have much less of an impact on you in the long-run.
Situations where DUI's are charged as felonies
Every state in the US at this point has adopted .08% BAC rule, meaning that if your blood alcohol content exceeds .08% when you are pulled over you can be charged with a DUI. Every state, however, has their own interpretation and different set of rules in regards to DUI's, especially whether they are charged as misdemeanors or felonies.
You can almost guarantee that if you take somebody's life while under the influence you will be charged with a felony DUI - that will probably be the least of your worries since you may be charged with manslaughter or vehicular homicide in this situation. If you are in any accident where you cause bodily injury to someone other than yourself you may be charged with felony DUI (this varies by state).
If you cause extensive property damage you may also be charged with a felony - this also varies from state to state, but if you hit another car, a residence, public property, etc. you will likely cause quite a bit of damage.
Some states now charge anybody who has been convicted of a DUI within 7 years with felony DUI if they re-offend. The idea is that somebody who has already been through the process and still chooses to drink and drive needs more serious consequences to really be rehabiliated.