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Hi Brian,
There are several criteria that discern between open and globular clusters.
Age, Metalicity, density and orbital characteristics are probably the major ones.
Globular star clusters are thought to be the first structure to form when Galaxies first formed. This leads to some interesting defining characteristics.
Being the first structures to form means that their stars formed from primordial material and hence are metal poor, as pointed out by Ron.
Their stars are also the oldest stars in their host galaxy.
With regard to spiral galaxies, globular clusters formed before the law of conservation of angular momentum caused the collapsing proto galaxy to spin down to a spiral shape. This is very important and infers that the globular clusters formed before the spiral arms formed. This has left the globular clusters on great elliptical obits above and below the galactic centre. Following on from this, or more importantly due to this, they don't share in the rotation around the galactic centre that the material within the spiral arms does.
Open clusters formed later in the spiral arms and hence tend to have higher metalicity and have stars at a much greater variety of ages. The densest open cluster is still not as dense as a globular cluster.
That there are mistakes in early catalogs is more an indication of the technology and equipment of the time after all many galaxies were catalogs as spiral nebulae.
That said there have been cases of merged dwarf galaxies being identified as globulars, there is also talk of young globulars, but the current thinking of that is far from being one of consensus. Even these so called young globulars are still very old, I think older than open clusters.
Hope this is of interest.
When doing my degree I wrote an essay on this very subject and although I probably should have dug it up for reference, from memory I think these were the most pertinent points.
Regards
Trevor
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