1) Bill tip pattern: American shows more red on lower mandible, with the yellow on the upper and lower mandible forming a wedge shape whereas Eurasian birds show a more or less straight line angled back across both mandibles. He cautions that Eurasian birds sometimes show the pattern of American birds.
I have investigated the bill tip pattern on the pictures I have of German birds (not many, maybe 5 or 6 birds in total) and on the galleries of the websites club300.de, tarsiger.com, birdpix.nl, and netfugl.dk. The sample size is thus around 60 birds. I have applied the qualitative character of the line on the lower mandible angling forward, as any quantitative character regarding extend of yellow on the lower mandible would be extremely difficult to assess in the field.
In doing so, I reached the following statistics:
Of all birds photographed in Europe, 70 % showed a pattern described by David Sibley as "Eurasian" while 30 % showed a pattern reminiscent of "American". As David Sibley points out himself, this character's significance as an identification criterion indeed appears to be rather small.
2) Shield size: American birds show a distinctly larger shield that is flat or even notched at the top.
I have not analysed this field character quantitatively as above. However, the shape of the frontal shield - as pointed out by Sibley - is very variable within Eurasian birds, reflecting age, sex and hormone levels of the respective birds.
While most Eurasian birds show a frontal shield that broadens only very little towards the top and forms an evenly rounded tip, some broaden significantly. Those broadening shields even tend to show a somewhat flattened top in that they develop slight rounded "corners". However, even in these broadest shields the highest point is always in the centre in the form of a little tip. A notched top of the frontal shield might indeed be a valid qualitative field character of the American form.
3) Eye colour: more or less bright red on Eurasian and much duller maroon or reddish-brown on American Moorhens. David Sibley states that the differences are small and eye colour is notoriously unreliable as a field character.
Most pictures of Eurasian Moorhen show birds with very dark eye colouration. The very few pictures I found of birds with rather bright irides were taken of breeding birds in June and July, suggesting seasonal variation (relating to hormone levels maybe?). As Eurasian vagrants are most likely to show up in North America outside the breeding season, this field character is of very little value - if of any value at all.
4) Leg colouration: both forms show differences in the amount of red on the upper part of the leg (sources differ on what form shows more).
Leg colouration is quite difficult to assess in the first place as the upper part of the leg, above the joint, is hidden in the bird's plumage most of the time. However, I concur with David Sibley that Eurasian birds seem to show less red than American birds, but that this is very variable and the stronger-patterned Eurasian birds might very well overlap widely with the weaker-patterned American birds.
I have included a few of my photographs of Common (Eurasian) Moorhen to illustrate my points below. All images were taken in Stralsund, NE Germany, in April 2009.
A "classic" Eurasian bird, with a small frontal shield (older immature or female?) that does not broaden towards the top, and an extensive yellow tip to the lower mandible. Notice however that this bird's eye is clearly very dark. 
This image shows a bird with a broadening frontal shield and a hint of "rounded corners". Note however that the highest point is in the middle of the shield, in the form of a clearly defined tip. The pattern on the lower mandible is clearly "American" in that it angles forward, although a classic American would likely show more red/less yellow. This bird's eye colouration is on the bright side of the variation shown by Eurasian Moorhens, although the eyes can get a bit brighter on some birds.
The bill tip pattern on this bird approaches the "American" pattern, and the eye is dark, appearing almost black (an effect of it being in the shade, compared to the previous bird). The shield clearly broadens towards the top and has rather well-defined "corners", yet still is clearly pointed at the top.
This bird also shows the "American" pattern on the lower mandible and a dark eye. However, the frontal shield is relatively narrow and clearly rounded / pointed at the top.
