Although overall I quite like the LotRO interface I agree with most of the suggestions on this thread. The biggy is lack of external plug-in support, but leaving that aside I think some of the current systems could use some tweaks.
I find the 'alias' commands great for crafting adverts, though i'd like to see them offer some combat uses. It would be a nice addition if it was possible to use 'alias' to allow targeting macros during MPvP and Raiding, furthermore the ability to assign 'alias' macros to icons which could be placed on one of the hotbars.
As mentioned in other posts, it would also be great if the 'ctrl + / ' command enabled different aspects of the UI to be scaleable.
On a final point (not sure how UI related it is) but I find the game camera very limited, LotRO probably has the best FPS mode in any MMO to date but beyond that its plagued by the same limits as other fantasy games. Anyone who's played Eve Online will know what i'm talking about here