Before I go into this, I must note that SD has warned people not to use mods with 1.5, and that what I'm saying here might just be something that has been fixed internally, or won't be an issue in the final release.
To get to the meat, I ended up running 1.5 with a mod, SoGE, out of curiosity, to see that it ran. To my complete and utter surprise, it worked, and even loaded a map without issue. At least, it loaded one random map fine, the second one caused an MD, for no apparent reason. After an educated guess and use of the ingame map maker, I figured out the cause of the second MD, DLC planets. To explain, the aforementioned mod has it's own Barren planet (PlanetBarren.entity which has the same file name as 1.5's), it has for some time; I made a map which had one of these, and upon hitting preview, I got an MD.
This makes me ask the question, does the existance of the DLC "lock out" entities with the same name as the DLC planets (PlanetBarren.entity, PlanetOceanicFair.entity, etc), as this will have an adverse effect on some mods.