Please read: The stack is an implementation detail, and don't ever again repeat the canard that stack allocation is what differentiates value types from reference types in .NET. The CLR may choose to allocate a variable anywhere it wants to.
The most important difference is in the assignment semantics. When you assign a value type to a variable (or pass it to a method as an argument), all of the data is copied. When you assign a reference type, only a reference is copied - both references point to the same object instance in memory.