I was playing the Rift beta when the following sequence of events happened, and got me thinking.
I usually put my crafting skills on the right-side action bar. Rift crafting skills have a "disenchant" option (which is pretty neat). For example, armorsmiths can "salvage" armor to get materials to make more armor. So I added a right action bar and put the Salvage button on it as I would in WoW. I then opened my bags to find some armor I wanted to break down. To my surprise, the bags opened in the same spot that they normally do, overlapping the right action bar and hiding the Salvage button.
This, of course, made Salvaging a rather tricky option. I figured out that you could move the bags around, but sadly, I couldn't get them to perfectly line up anymore. So now when I open my bags, they display in a rather raggedy manner, but at least they're not obsuring the action bar.
I started wondering why something like this never happens in WoW. I realized that--for all that WoW and Rift are very similar--the default WoW interface uses a completely different windowing system/metaphor than Rifts.
The default WoW interface uses tiling windows. For the most part, there is only one plane, and all interface elements exist on that plane. Nothing overlaps, and if you open a new window, the other windows either shift position or close in order to accommodate the new window.
(The preceding paragraph is not strictly true. There's really two planes for the default user interface. The lower one (which can be obscured) has the group/raid health bars, the quest tracker, and the small zone map.)
Rift, on the other hand, uses an overlapping windowing system. Interface elements can overlap and be moved around, or brought to the foreground at different times. But the price you pay here is that if you want to see two elements at once, you often have to rearrange things yourself.
Just a small technical observation. I think I like the tiling system better for a default game UI. It's probably more work, making sure that all the elements tile nicely, especially related elements. But everything that is active is immediately available to the user, without them having to move elements around to see information.