Let's talk about what you need to know, and (more importantly) DO about the upcoming Winter '19 release. This action plan takes a look at some of the more generalized features. Check out the Sales Cloud guide as well when you're done.
Flow Solutions - there is a new step for you to take as you assess solutions to your businesses needs. Before you build your own flow, check the new flow solutions section on the app exchange to grab automation that is pre-built!
Display Density - This can be set individually by users, but don't forget to set your org-wide default setting. Personally I would recommend setting it to "Compact" as I find it to reduce scrolling and improve the user experience. Just go to setup in lightning and search for "density settings" in the quick find to set your default.
Quick text and Macros - Make sure you go to the settings for both of these and enable folders, as this will bring organization to both tools.
Change Owner in List Views - the long awaited and needed ability to mass change up to 200 records owners at a time on cases, leads, and custom objects is at last here. Make sure that the action is available to you users, that any custom actions or packages that you may have been using as a workaround have been removed, and that proper permissions are in place for your users.
List View Sharing in Lightning - Now that users in lightning can share list views(thank goodness), you will want to check your permissions. If you were letting users switch back to classic to do this in the past, look at if you can update their profile to keep them in lightning. Additionally, if there are user groups that you don't want to share list views, you will want to review their permissions in case they were previously only limited by the limitation in Lightning.
Lightning Data Packages - a very important feature is the ability to manually choose a match from your data provider, when the highest confidence match is wrong or the correct match is below the matching threshold.
Lightning app visibility filters - you can now reference custom user permissions when defining visibility of a lightning app. Think about any workarounds you might have used on this and cleanup your lightning filters as applicable.
Event Series (Beta) - If you start trying the recurring events in lightning that are now in beta (just don't), there is a laundry list of items you need to be familiar with. I wouldn't play with this yet unless you are staying extremely vanilla in terms of fields, as one of the limitations is that you can't save to custom fields on the event.
Talk to your developers
Background Utility Items - If you have savvy developers that are on the forefront of lightning, they have probably done some neat things in terms of lightning components that listen for events, perform a function, but don't actually have an interface. You previously had to create little components that existed on the lightning page or in the utility bar. These can now be updated so that they are no longer taking up space in those places, and simply run in the background. There are also some new features to support privacy legislation you can check out here.
Custom Screens - If you are using the screens lightning component, there is a new elegant way to add custom validation before proceeding to the next screen. Use the code snippet found here as a guide.
Deploy flows or process builders active - you can deploy these as active into production using the metadata API if you are running a continuous integration environment.
Help your business leaders and executives
Data Retention - Salesforce has a useful guide on how to comply with data retention policies that may affect some or all of your business depending on where you operate. Many of their solutions require work to be done via API currently, and you do not want to get stuck doing that for each and every customer request that comes in. Proactively start planning to build and automate workflows for data removal.
Field History - Technically Salesforce has always only guaranteed that they would keep field history data for 18 months, but it has been a not so secret fact that they retain it well past that limit. They are tightening down on this, so if you have field history you need to preserve for longer than that, talk to your leaders about purchasing the additional retention.
Inform your users
It goes without saying that for all the items above, you will need to inform you users of the changes that you make, or changes that will affect them in the new release. In addition, here are some items to ensure they know about.
Browsers - If you have users in classic or a VF + Tabs community, they should know that support for IE 9 & 10 is ending, and that they will need to use a supported browser going forward.
Searching in a list view - make sure your users know that when they use the new list view search, the scope is restricted to the list view. Otherwise, you may get complaints of a record not existing, when they are searching in the wrong place and think the search is global or object wide. And while they can search fields that aren't visible in the list view, there are certain fields that are not searchable that you may want to advise them on so they know to filter or sort on those instead.
Chatter post drafts - if you are working on a chatter post and navigate away, it will now be saved as a draft!
Mobile updates - there are many new small enhancements to the Salesforce app, my favorite being the visibility of approval requests across the mobile experiences.
If you have other questions or need help with any of the new features in this release, please feel free to reach out to me and I am glad to help. Reach out to me on LinkedIn at linkedin.com/in/spencerklowe or enter your information anywhere on this site and I will get back to you.