As per discussion in the VDG, currently missing features in the overview effect to be able to replace the other effect(s): 

-- needed before replacement

* Show only windows of a specific desktop
* Show only windows of a specific application

(With keybindings each) 

These are needed because e.g. task manager has an option to use the effect in which the windows of a grouped application are shown 

* Possibility to set a flag to disable the virtual desktop bar on top

needed for the above, mostly, since it doesn't make sense to have a desktop bar in this situation 

--- would be good before replacement

* Possibility to configure mouse buttons and their effects, mainly for pulling a window to the current workspace, but also for people who want to disable closing 

* show the window previews of the windows matching the filter you just entered in the text field 


Present windows also has some other options, e.g. ignoring minimized windows and a more fine grained configurability of placement and presentation, not sure if / how often these are used. The above are more functionality than aesthetics and I think at least some of them are a must have before present windows can be replaced.