Does it drag in other KDE 4 dependencies?
Obviously it depends on kdelibs and kde-runtime like any KDE application. The dependencies will of course be better once frameworks 5 are released.
Considering the impact on a running environment I would say that all depends whether your users are using any KDE 4 applications or none. I would be surprised if your users would use no KDE 4 applications. So I think it is negligible.
Right away AFAIK we would lose DCOP integration, the window styles that come with TDE, and the kcontrol integration. Users can already run different window managers (compiz anyone?) so I would say this decision should be made on a per-user basis. A set of instructions on the Wiki for configuring a users' session to use kwin4 wouldn't hurt. ;-)
Tim