Obtaining a good word list is hard, I have one saved which is a merging and editing of several freely available ones, including one used in Scrabble competitions and one from some Linux distro, and a few "basic word lists". There will be endless errors like missing plurals and past tenses, as well as the inclusion of things like acronyms that shouldn't be in there at all. And if you prune obscure words but allow the submission of missing words, I guess you'd better keep the list of pruned words around so you can head people off at the pass by pre-filtering their submissions.