Think that tab should be default key for completing AutoCompletion. Enter is to much in the way. When you write large document and have language with a lot of cases, Enter fails. Matching first four letters doesn't mean that the same case will be used, so AutoCompletion is suggestion _wrong_ word (if we are going to be pedantic). So if you want to start a new paragraph, AutoCompletion kicks in and then you need to go back and correct, and AutoCompletion kicks in once more and it suggesting the same word once more and it's wrong once more... Enter is not good choice. It's a great feature but it's too intrusive set like this. Thanks
Let's ask design team. Changing such a default is very dangerous.
The dialog Tools > AutoCorrect > AutoCorrect Options... allows to change this setting under Word Completion. You may "Accept with" return (default) but also per end, tab, right, or space. => WORKSFORME Read also https://help.libreoffice.org/4.1/Writer/Fine-Tuning_the_Word_Completion_for_Text_Documents Btw, whether Enter or Tab depends highly on the tools (and OS) you are using. Developers know tab from their IDE, but text is usually _entered_. Remember, for instance, how email addresses or hyperlinks are completed.