Recently, I've been asked what the difference between the "Leiden school" and "mainstream" Indo-Europeanists is. The asker is planning to study in Leiden and has been concerned with the many vague criticisms he has encountered in connection with the "Leidenites". I've noticed the differences mainly pertain to the (methodology of the) reconstruction of Proto-Indo-European (especially in connection to the accentuation, "glottalism", Germanic substrate theories etc), but there seems to be no detailed, coherent, comprehensive list of the particulars, just bits and pieces scattered here and then. Could somebody possibly make a list like that here, or provide suitable references? It might be both useful and interesting to see what differences there are between other schools and, within the "mainstream", between its "undercurrents".
Leiden school are people who propose some strict rules for PIE and strictly adhere to them. Strict root structure, no vowels except /o/ and /e/, three laryngeals etc.
Their opponents are those conservatives who either skeptic about some of the Leiden rules or those who supports more loose rules (for instance, vowel /a/ in borrowed words, 4 laryngeals, some roots with two vowels etc).
In short, Leiden is for extensive, numerous, strict, regular rules, their opponents are for less rules and more exceptions.
Presumably 'the Leiden School' refers to the views of the professor (who is respected) there. To discover his views look at Kortlandt, Frederik 'Studies in Germanic, Indo-European and Indo-Uralic' (Amsterdam, 2012).
Every University that teaches Indo-European studies will have a leading professor who, if he's worth his pay, will have his own views on the subject.