In the original version, why doesn't Mumm-Ra get more proactive about dealing with the Thundercats, instead of lurking inside his pyramid all the time? It turns out that anyone can steal/use the powers of the Ancient Spirits of Evil if they gain access to the appropriate chamber. Mumm-Ra doesn't dare be away from the pyramid any more than absolutely necessary.
The mutants have perfected a weapon that will burn down any area they choose. Why bother waiting till sundown when it's already locked and loaded on Cat's-Lair right now? Seems like a case of villain ball until realizing that by not making a move till nightfall, they will not only destroy their enemies' headquarters, but also most likely immolate them all in their sleep.