As of right now msn adcenter works only in msn search, therefore it is a solution for advertisers not publishers.
There are plans to make it a publisher solution. However, yahoo will have learnt a lot of the pitfalls that google encountered and msn will also take all this on board. In theory it should mean better quality of websites get accepted, there are more stringent conditions for acceptance and that overall payment for publishers should be better while advertisers are more protected from abuse. That is the theory, but it could be much the same as adsense with as much confusion and inconsistency.
Well, I don't think that Yahoo only allows sites with over 100K views/month. My website got accepted by Yahoo Publisher and I am well under that.