The concern about memory usage is valid. I think its about changes in the X stack. Many components have been deprecated and new ones have been introduced. The changes are, in general, at the API level. That means that allegued improvements are for Desktop Environment implementors. User should expect better stability, faster fixes (with better APIs programmers can focus in fixing things that matters.) If this actually happens, idk. We have seen new APIs for user session handling (consolekit), IPC (dbus and friends), resource and power management (udisk). These components are in some cases optional, if you make the right decissions (like using a window manager that not depends on them.)
In a reflexive tone, I ranted about this a lot (remember HAL?), maybe its time to do our research and decide if we want to avoid them and how. In principle, it is doable because I did it for years by using an alternative window manager, mounting my disks when I need them, picking apps that do not use dbus, etc.
In mobile devices its harder, because networking applets depend on dbus, power mangement requires extra daemons, etc.
A sidenote. Keep in mind that estimating RAM usage is hard and most applications report just a close number. Also, RAM usage is affected by many components, so we do need to do empiric research to know whats exactly going on.