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[Manos: The Hans of Fate]

I'm sure it varies by state in the US, but don't you have to be witness to a pretty severe crime to justifiably perform "citizen's arrest"? I'm pretty certain a guy trying to leave in his vehicle does not constitute "a severe crime" regardless if he was trespassing or not.

In general yes, but as you mention there is great variation

The Wikipedia page has a pretty accurate general description, though it lacks mention of shopkeepers privilege.
http://en.wikipedia.org/wiki/Citizen's_arrest#United_States

[United States
Each state, with the exception of North Carolina, permits citizen arrests if the commission of a felony is witnessed by the arresting citizen, or when a citizen is asked to assist in the apprehension of a suspect by police. The application of state laws varies widely with respect to misdemeanors, breaches of the peace, and felonies not witnessed by the arresting party. For example, Arizona law allows a citizen's arrest if the arrestor has personally witnessed the offense occurring.[37]
American citizens do not carry the authority or enjoy the legal protections held by police officers, and are held to the principle of strict liability before the courts of civil and criminal law including, but not limited to, any infringement of another's rights.[38] Nonetheless many citizens' arrests are popular news stories.[39]

You can also arrest (general rule) someone if you know a with certainty that a felony had been committed. If there is a mistake to the occurrence of a crime, there is strict liability for false imprisonment, as a tort at least, but an allowance for a defense for reasonable mistaken identity is generally allowed, but not the crime itself.