Across West Germanic Languages, what sound changes have been most common since 1000 CE? For example, has there been much epenthesis (vowel insertion) or syncope (dropping middle vowels) or metathesis (transposing sounds, as in flutterby becoming butterfly)? Other historical processes?
This chart at Wikipedia gives an overview of the consonant changes of Low German compared to other West Germanic languages and may be helpful to you.
For sound changes that happened after ca. 1000 there are some candidates:
- th-stopping: Essentially all continental Germanic languages stopped their th's to d's, including Frisian
- unrounding of umlauts vowels: /y/ -> /i/ and /ø/ -> /e/ happened in English, Yiddish, and is frequently encountered in German dialects
- apocope of final e/schwa: Still an active process in German dialects
- diphthongisation of long vowels: /hu:s/ -> /haus/ and /mi:n/ -> /main/ happened independently in High German and English
Well, since 1000 CE... That makes it a lot more difficult. I'm not sure if any of these changes may have happened earlier.
Reduction of unstressed vowels to schwa. Happened in English, Dutch, Low Saxon, German and more languages.
Loss of gemination.
Regarding the diphthongisation of long vowels that jknappen mentioned: That happened in Dutch too (didn't happen in Low Saxon, Limburgish/Ripuarian Frisian(except open syllables), most Alemannic German). mîn- en /maɪ̯n/ de /maɪ̯n/ nl /mɛi̯n/, lûs- en /laʊ̯s/ de /laʊ̯s/ nl /lœy̯s/.