DQIX multiplayer is not like Diablo in any way.
Let's see...
the other players are taking experience that could be going to your normal non-human party members
Since the game can feasibly be completed with any sort of party, including one consisting of no one aside from the hero, this is less of an issue than you're making it out to be. Especially since said hero is getting
more experience playing with others than he would playing with his own three party members, because experience gains for everyone get boosted when you have more players in an instance. You know, like
Diablo II. Unlike that game, though, in DQ9 the levels themselves are less important than the skill points earned from leveling up, because levels don't carry across classes (not that D2 has class-switching) (yes, I'm aware that leveling in D2 is a very different beast from leveling in DQ9).
the fact that multiple humans taking their own actions will have less of an overall cohesive strategy than one human controlling all 4 party members (at best, equal cohesiveness).
I'll grant you that different players will probably have different strategies, but the same goes for pretty much any co-op game that doesn't have strategies of the "this is the only thing worth doing" variety, and in this it'd rarely be anything less than equal cohesiveness, because it's pretty easy to coordinate when it's turn-based and you can only do local wireless. And different strategies could be a good thing; as a very basic example, one player might have advanced their hero in a class the others didn't, and be able to use skills the rest can't as a result.
only the host player can make storyprogress
Like
Diablo. I think this is actually a good thing for a story-based game like
Dragon Quest, so I like that it's set up so that only the host player can initiate story events, rather than, using
Diablo II's first act as an example, it being possible for a high-leveled guest player to go kill Andariel while the host is still in the Den of Evil.
leaving any other players to take their now-overleveled single character back to their "real" game
Like
Diablo. Except in this it's less of an issue, since, again, skill points are more important than any levels earned through experience.
I do agree that DQ9 multiplayer has major problems (including some of the things you pointed out), but many of them aren't just problems with this game, and at least this game tries to deal with them. And because you can in fact have different character builds, having someone bring their character into your game is a lot different from them just taking over control of one of your own characters.
For what it's worth, I think the fact that you instantly gain access to the downloadable quests by visiting the world of a player who has them is a bigger problem with the multiplayer stuff at this point in the game's lifespan than anything the multiplayer combat changes or doesn't change, because it means the only reasons to update DQVC are the legacy characters' outfits (which I hope are available some other way) and the offchance that one of the sale items it generates for you that day is mini medals.
What meaningful additions does DQIX have that DQVII or DQVIII didn't?
The main one I can think of is non-random battles (which DQ8 sort of did, but only with the special monsters for the arena;
Dragon Quest Monsters: Joker did it for everything, but DQ9 is the first standard game in the series to have regular field encounters be that way), which really is a bigger deal than you'd think. And tied to that, while not doing the whole "you win low-level battles instantly after a certain level" thing
EarthBound does (it can't because of the sidequests), DQ9 does have weaker monsters in the field run away from you if they see you, so you can chase them down if you need to, or just ignore them. I believe
Chrono Trigger did this, and know the last two
Persona games did, but I can't think of anything else. They also expanded heavily on the alchemy system from DQ8, but this could also be seen as a bad thing, since the game has over a thousand different items, and you can only find certain recipes through trial and error.
Bringing FFXIII's characters into the discussion seems weird because DQIX has NO characters.
I'm sure Chupperson will disagree with me here, but I think not having characters with personalities is better than having characters I can't like, even though the latter is the "modern" approach. (Actually, the true modern approach is pretending you're making a WRPG and/or a dating sim instead of a JRPG when you're writing character interactions, but Atlus is the only company doing that.)
It's important to note that in none of these posts I'm trying to say DQIX is a "bad game".
Nor am I saying you are. My apologies if it seemed otherwise; it may very well have given how often you attack the genre at hand.
I'm saying DQIX gameplay philosophy is roughly on par with the FFIII era, and thus Tycho is a hypocrite regarding SCII.
And I'm saying that DQ9 as a whole is closer to this decade than SC2. And as I said before, while I don't necessarily agree with Tycho's opinions regarding the game (particularly anything he said with regards to editors, since the SC2 editor is a lot more versatile than the WC3 editor, and is a part of the game where his sentiments simply do not apply), I do believe it's right to say that the end result after twelve years being little more than a remake of the original is a bit disappointing.
I do think [DQ9] was hurt by their multiplayer fixation in two ways: it necessitated the aforementioned lack of player character personality and caused the game to be on DS, which nosedived the series' graphics. (Not that DQ cares about graphics--look how it stayed up through DQVII--but DQVIII was just so gosh darned pretty...)
While I do agree that it being built as a multiplayer game hurts it in terms of them designing a central cast and in terms of some of what they could have done with the gameplay, as you said,
Dragon Quest isn't about the graphics (though DQ8 is indeed a beautiful game, and I believe DQMJ looked better than DQ9), and it being on the DS adds the very wonderful feature of it being portable*. I do have a different aesthetic issue with the same cause, though: while Koichi Sugiyama's musical compositions are excellent as always, their rendition as digital audio (aside from the overture) rather than live orchestra recordings as in the previous game is rather depressing (particularly because the OST is the same way).
*I am firmly of the opinion that JRPGs are a far better fit for handheld systems than any console. While you obviously can't do something with the graphical intensity of FF13 on a handheld, I find myself increasingly more likely to play a handheld game than a console game. For me I guess it's something about being more capable of putting extensive time into a game I can take with me anywhere than a game where I'm tethered to a television.