The new 3rd Anniversary interface and the Gold interface both have features not present in the other UIs, namely:
- The selected target shows an HP + MP bar, with class information.
- The action slot has an indicator for which slot is currently being cast, which provides valuable information about when it will complete.
The other UIs, such as Masquerade, Original, and Mobile, do not have this information. Mobile may be a special case where it's not practical, but the Masquerade/Original UIs need to be improved to add these pieces of information.
It's a literal disadvantage to not have them.
Please bring feature parity between the various browser UIs. 
Somewhat related:
- The modern UIs take way more space than the original UIs, please no...
