Update: this is fixed in OS X version 10.10.1

if you are on Yosemite and use Dropbox, please make sure that you go to Dropbox Preferences - General and uncheck 'Enable Finder integration'.

While this will get rid of the fancy green check marks in the Finder it will also stop crashes when displaying File Open or Save dialogs. Dropbox will still operate normally.

Thanks to the folks at Dropbox for describing the work around. Their description is here:

https://www.dropbox.com/en/help/7144