I've only played this game solo and so far only with the 4 default decks, but it seems to me that the game is going to be a bit easier the more players you have (I guess that is only natural, to be fair), simply because of the number of extra characters you have to deal with stuff. Sure you get 1 extra card into the staging area, but you get 3 extra characters to deal with stuff, which is a net increase of 2 over solo players, per extra player.
I'm not complaining about this, because the difficulty of the solo game seems about right to me (its certainly not easy!), and if it gets a bit easier with more players, perhaps thats a good thing as it makes the game more about a social activity with friends without the 3 round massacres that I have witnessed on a couple of occasions solo!
I also think that a solo deck is going to be built totally different to a co-op deck, again not a bad thing.
From my experience, when playing solo it is all about how many characters you can get down on the board, to allow you to get enough willpower to devote to questing, while still keeping enough characters back to deal with the combat phase. I have had more success so far with the leadership deck (count the number of allies in that deck compared with others) than the other three, and Aragorn plays a big part of that i think, as does Celebrian's stone, for the willpower boost.
The lore deck seems particularly tricky, because although they have lots of support actions, especially around healing, this all becomes a luxury in solo mode, when what you really need is to remove encounter cards from play to keep the threat levels down. Lore does have forst snare which is an excellent card.
So far, i think a solo deck will end up being a primary leadership deck and secondary a.n.other deck depending on your play style preference or favourite heor (Eowyn from spirit again has high potential due to her high willpower and hence questing power).
I see a tactics/lore deck combo struggling in general terms, because the tactics allies are poor at questing to say the least. I can see this kind of deck being able to hold its own without making much progress but in the end it will be overwhelmed by too many locations in the staging area, unless the player gets lucky.
However, in a co-op game,, a tactics/lore deck would make a very good support deck for someone elses leadership heavy deck.
What have other peoples experiences being with playing with the default decks, or with more or less players?