Is there a fundamental difference between a small galaxy, and a large star cluster? For instance, If a large globular cluster were somehow ejected from its galaxy, and was observed by astronomers, would they call it a galaxy, or would there be characteristics of the star cluster that make it obviously not a galaxy by definition? And why are the Magellanic Clouds dwarf galaxies, and not large star clusters? Could we remove stars until the Magellanic clouds became star clusters, or is there something intrinsically galaxy like about these objects?
There are a couple of important distinctions between the two types of objects. Galaxies are objects which range in mass from about $10^{9}-10^{12} M_{\odot}$, and contain 'halos' of dark matter which represent the majority of the mass of the object. Now, there are things called dwarf galaxies (which are less massive than regular galaxies; I'd imagine that there really could be a bit of a blurred distinction between these two kinds of objects), however dwarf galaxies still seem to have dark matter components to them.

Globular clusters are tightly bound collections of about $10^{6}$ stars, and do not have significant (if any) dark matter contained within or around them. So for these reasons in practice it is not hard to confuse the two.

Some References:

1) Evidence that dwarf galaxies are dominated by a dark-matter component. 

2) Dark matter present in globular clusters are either unlikely, or incredibly highly concentrated and approximately the same mass as the stellar component.