Compatibility, and Stability
All Mod packs should be unit tested, and then tested with other packs. This consumes time and labour. And there should be test reports and stats and graphs- pages of them.
A mod author should be able to vote for inclusion or exclusion of other mods in the modpack that bundles his mod. That probably means that if his 5 star job gets stuffed in with a bunch of 2 star jobs, the ctds will be blamed on his mod just as much as the others. Will he enjoy providing Support for the Bug Reports?
Mod Packs will then want to be rated on test results as well. A extra burden on those Nexus staff and devotees who want to give it any credibility.
Demands for Modpack Compatibility
This is the worst thing- that a mod should be altered without permission so it functions in some mod pack.
Mod Authors have every right to determine how their work is distributed!