I can also say, "Dang, I can't run HoD with my existing KotHH deck". So saying "I can't run KotHH with my HoD deck" is a silly argument. It goes round and round with all of the agendas. The opportunity cost of running an agenda relative to choosing any other agenda only has value in the inherent power level of ranking agendas. EVERY SINGLE AGENDA HAS AN OPPORTUNITY COST!!
When you add in the fact that I believe that the Maester, Summer and Winter agendas are overpowered and need to be banned to refresh the metagame, then yes, HoD is a problem because it has a power level comparable with those three.
I agree. Just like every card in a deck has an opportunity cost; it just happens to be more significant in the agenda slot (and House Card slot and restricted card choice). It isn't a silly argument, it is part of deckbuilding that limits what you can do with your deck. The rules embodiment of opportunity cost is in the restricted list.
I agree with the overpowered nature of some agendas (and themes), but I don't think HoD is comparable to those agendas you mention. The simple reason is that you can still have that powerful location in those other decks, while you cannot have those agendas in your HoD deck. It is very good, but until we see play how do we really know. I personally do not consider it on par with TMP, Kings of Summer or Kings of Winter, but I might be wrong.
My expectation is that you will see it in a ton of decks that previously ran no agenda (or chargenda). I think it will also do a lot to flesh out the field - I'm looking forward to rebuilding my Stannis nedly winter Wall deck that I stopped playing because I couldn't play a 4 gold location consistently. Yes, people will find out the best locations to run with it - those Bear Island decks will be losing Siege of Winterfell and those Aegon's Hill decks will lose Kings of Summer - and we will see optimal HoD decks for every house. What I expect from FFG is a bunch more agenda related cards (like Daemon/NCF/etc.) to balance the use of agendas. At least I hope.