In particular, I think Diablo 1 was every bit as item oriented as 2, or more, as skills depended on finding books in the dungeon.
This is both true and not true. Yes, you needed spellbooks, but they were relatively available; you didn't have to grind for them. You could buy them at Adria's when you have money, or find them in the dungeon fairly easily most of the time (the early levels have a bunch of bookshelves in case you're unlucky). This is item-oriented in the sense that you upgrade yourself using items, but not in the sense that you have to spend a lot of effort getting them; you get items while playing the game, as opposed to playing the game just to get the items. There were few ultra-rare spellbooks, and you weren't killing the same monsters or delving into the same areas over and over again to get them.
When it comes to actual equipment, it's obvious that D1 is less item-grindy for a number of reasons. First, unique items were less all-powerful, so finding specific ones wasn't that important (surviving without The Right Equipment in D2:LoD is quite hard on higher difficulty). Secondly, the modifiers you found on items were fewer in number (1-2 per item, maybe a couple more on uniques, whereas in D2 you had much, much more) and often lesser in power, so they didn't matter as much; having a shitty axe instead of an uber-axe-of-the-powergamer didn't outright ruin your character. Third, the good items you could find were ones you could actually find through normal gameplay, nor was there even an efficient way to grind in the first place. In D2, there are very rare items that everybody wants and that don't drop often and there are certain places to grind in order to do this. In D1, those grinding options don't really exist very much even if you wanted to do that, and it's not necessary to do so anyway, because pretty much anything can drop anything assuming you're at the right dungeon level... and like I said, which items you have doesn't matter as much to begin with.
Basically: In D1, it's completely feasible to just play through the dungeon, gather up the items you find, and rely on those even if your luck isn't great (supplementing it with stuff you get from Wirt). Diablo II
starts off this way (in Act I Normal you still might actually pick up a white non-magical item! Wow!) but quickly diverges from that, damn near forcing the player to replay boss fights until they get enough Nice Neat Shit to survive later on.
If you ask me they should have expanded on the roguelike aspects of Diablo 1, instead of going more arcade. Keep the base character and skill system as it was, only add more backgrounds, maybe even expand on DIablo 1's character special abilities and make it into a proto-skill tree (but with the book system still in place as the major skill system), and set this on a larger world, in which you have to explore to find out stuff, and travel from settlement to settlement.
I can totally understand the design decisions that went into D2, or at least I can see how they could have happened, without them necessarily being aware of how they would affect the game. I could see them having thought "Unique items and random magic drops are interesting; let's expand upon that with item sets, 'rares' with more modifiers, more interesting uniques, item sockets, and things like that", for example, without necessarily realizing how much that would change the very gameplay paradigm they were dealing with until it already started happening.