After opening a Word, Excel or PowerPoint file, you'll be able to make changes, watch others make changes and chat through comments in the same document, spreadsheet or presentation. All of the changes get saved to Dropbox and you can keep working even if all of your coworkers close the file. The new co-authoring feature is available today for Basic, Pro and Business users.
Dropbox is looking to ease email file attachment woes as well. The company says that users will be able to send massive files in Outlook, no matter how large they are, in the coming weeks. Details are a bit scarce for now, but this sounds like the feature that Google Drive allows Gmail users. With that setup, you can still offer the recipient access to a file stored in Drive, even if it's larger than Gmails size limit.