It has been consensus for a while that Scala's current collection classes leave some room for improvement. They sufferered from inconsistencies and some lack of functionality. The main problem was that there was no unique design principle supported by a framework in which individual collections could be easily integrated. Therefore, contributions over time increased entripy and caused various inconsistencies.
The current proposal is an attempt at a uniform design, which maximizes sharing and code reuse. The ambition is that every piece of functionality should have one and only one logical place to reside. We are not quite there yet, but we are already quite far towards this goal.
Links:
[1] http://www.scala-lang.org/sites/default/files/sids/admin/Tue, 2010-07-20, 10:39/collections.pdf
[2] http://www.scala-lang.org/sites/default/files/sids/admin/Tue, 2010-07-20, 10:39/collections.zip