14 DAY TRIAL // Ubuntu 14.04 LTS (Trusty Tahr) will be released with Unity7, the same one from Ubuntu 13.10, but this doesn't mean that it can't be improved. Canonical has just pushed a major update to Unity7 and it's likely that more are incoming.
A lot of users have been hoping that Ubuntu 14.04 LTS would get the new Unity8 that is currently being developed for Ubuntu Touch. That would have been great, but Unity8 is far from being complete and it would have been hard to promote it in an LTS version.
Ubuntu developers are now improving the “old” Unity7 version and have pushed a lot of changes for Ubuntu 14.04.
One of the main changes is the introduction of Unity Control Center, which has replaced GNOME Control Center. For now, the two versions are visually identical, but that might change with new releases in the future.
Also, a new combination of keys is available for locking the computer, Super (Win key) + L. This is the same combination used on the Windows platform, and it makes sense to have multiple combinations doing the same thing.
Other changes include multi-monitor support for EMConverter in Unity settings, Quicklist and Tooltip active blurring has been enabled again, the overlay window buttons now take multi-monitor into account, and the shutdown dialog and shortcut overlay are now transparent in Low GFX Mode.
Ubuntu 14.04 will certainly be a very interesting release, although some users might object to the fact that it won't be different enough from the previous versions. Visually, Ubuntu 14.04 LTS and Ubuntu 13.10 won't differ all that much, but the new version will feature a lot of smaller improvements.
Also, it's unclear whether this large update will also be backported on the Ubuntu 13.10, but most likely it won't work, because the systems are very different and Ubuntu 13.10 only has a few more months of support.
More details about this Unity7 update can be found in the official changelog.