I think it's great that words can be added to custom dictionaries; but it would be even better to be able to import entire custom dictionaries. Writing a scientific article for example in biochemistry inevitably leads to an awful lot of new additions. I tackled this in Word pretty easily with the fruits of another's labours, importing an entire scientific dictionary: http://www.jpetrie.net/scientific-word-list-for-spell-checkersspelling-dictionaries/

NOTE: I've added this as a new idea, as the previous spell check idea, on which I've posted a comment, isn't as specific and has been marked as completed