The biggest challenge I have found for most Lightroom users is the Catalog for a multitude of reasons. The way that Lightroom works is that files are imported into Lightroom’s catalog and the images are in a directory on a hard drive. The general consensus is that we do not want to store our images on our internal hard drive because the drive will fill up and slow our computer down. So a good workflow would be to store the images on an external hard drive and Lightroom will ‘point’ to the images. in theory you can have several hard drives that Lightroom can access and the files. This also means that you any external drives would need to be plugged in so that Lightroom can ‘see’ the files. If you were to unplug the drive then thumbnails would have questions marks and it would say the file is ‘offline or missing’. The hard drive would always have to be plugged in for you to access the files to work on them.
One of the best reasons to upgrade to Lightroom is the new feature of ‘Smart Previews”. Now when Lightroom imports files you can have it build ‘smart previews’ which will allow you to work on your files without having to have the hard drive plugged in. You can edit your images and then when you plug the drive back in all the metadata and changes will update.
This a welcomed change for Lightroom users as many of us are working on laptops and we want to be portable but if we have to lug around an external drive it doesn't feel so streamlined anymore.
Smart Previews can also be made for files that have been imported using previous versions of LR.