Another point you didn't mention is that posts after the first few replies are much less likely to receive any merit as they won't be seen much, maybe it is good in the sense that some of those huge useless threads will be less lucrative to post on but it also gives less incentive to continue posting on good discussions.
As for distribution of merit, we can already see from the stats theymos posted that most of the merit is going to a few well known people in the community rather than being distributed evenly to good posts. Also it seems this meta section has been the one section where most of the merit so far has been distributed to.
Theymos also asked people to merit posts of high quality rather than posts they agree with but just as with likes on reddit this will never happen.
Theymos has said he'll be monitoring it closely and adding more merit sources as he sees fit, hopefully that will be the case and we won't end up in a scenario where ranks are much much harder to come by. I don't think anyone will have a major issue with a slower rate of progression, even if it was halved or so, but if it's 50 times or more then it's a bit ridiculous.
I think that's something that will need addressing and I'm not entirely sure as to how that can be done, hopefully in time if there are more sMerits to be spread around then people can reward good quality discussion. As you mention in your next point it's important that they're used to reward people who are challenging one another and learning instead of just the people that you are in agreement with.