[...]Speaking of that, are there any serious gameplay differences between SP and MP in D2?
No. It's the exact same world size, number of maps, with randomization thrown in, such as the map's layout, placement of mobs, types of mobs, placement of interactives such as chests and shrines, etc, but the actual number of maps is the same, and some of them are never randomized in terms of layout, for instance the Sanctuary in Act IV, its layout is always the same, but inside it the enemies will be randomized. The quests are also the exact same. For all intent and purposes, it's the exact same game, with the difference being that if you happen to play Ladder (on-line you can play non-Ladder, or Ladder) then you'll have the possibility to access Ladder-exclusive items (unique items), Ladder-exclusive Runewords (via crafting) and Ladder-exclusive Horadric Cube recipes (still related to crafting). Those Ladder-exclusives are good, yes, but they are (mostly) very difficult to come by on your own, but since it's on-line you'll inevitably get some help, sometimes, if you're lucky (to get those items I mean, the Ladder-exclusives that is).
Additionally, the on-line economy from within the game itself is basically non-existent, and by that I mean that the "currency" are the Runes, and not any Runes, you must have at the very least some wanted mid-level Runes and of course idealy high-level Runes to trade for almost anything. I say "almost" because there are some exceptions, of course, and some players can be tolerant and accept other "valuables" such as item-to-item trading, for instance trading some item you found, which happens not to be a Rune, say... a mid-level unique item, for a similarly valuable item, so in the end no Runes were traded for anything. If not for non-Rune items (such as uniques or set items for example) then you can sometimes (I.E very rarely) manage to get some "cheap" (in terms of value) items with Gems (usually Perfect Gems, which fortunately aren't hard to come by starting from the Nightmare difficulty).
Truth be told, playing Diablo II on-line can be (and usually is) harsh, the items rarity system is very good, but the items drops are available to everyone, which means that it is a first-arrived-first-served system. If you play with "friends", or strangers (which will most likely be the case) there WILL be a "race" to get to the items first. A player who is familiar with the principle will always play around with the items highlight key at the ready, constantly on guard and ready to run at and loot the dropped items as you'd play with him/her/them. I for one played the game for years, many years... BUT... THAT very specific part of the game has always been my personal complaint. It IS contradictory to pretend that the game can be played in co-operation when in fact the only co-operation that occurs if it does is that sometimes players will target the same mob(s) as you do, and the co-operation stops right there. The game is NOT "friendly" when it comes to looting IF you play with other players on a regular basis, you WILL lose items simply by not having the time or the reflex to loot the items first and you WILL be frustrated by that even if you play it for years, the bad taste of seeing "your" items getting leeched or "stolen" always sticks.
In Diablo III, as often stated by Blizzard, the loot table is player-specific, which means that in a game in which four players play together they actually do play "together", completely, not just partially, simply because if you kill a mob, or many mobs, and one or more items drop, then YOU and ONLY you will see and be able to pick up those specific items, other players won't even SEE them, at all, and the same applies to you, you won't see "their" items, and THAT part of Diablo III is the ONLY reason why I do want to buy it and play that one over Diablo II, because otherwise I'm pretty sure that Diablo II would have remained the king in comparison (although that is still just speculation on my part).
So, "co-operation" in Diablo III will be existent in all senses of the word, in Diablo II however it really depends on the player(s) themselves with whom you happen to be playing. If you play with someone who really doesn't care that much about loot or because that player already "has it all" (which basically doesn't exist in Diablo II) then perhaps, yes, such a player could possibly just "ignore" the items as they drop, as he/she helps you around and let you pick them up, but don't expect such a player to actually TELL you what he/she found on "his/her own" as you both quested around together. There IS hypocrisy in Diablo II but there ARE "mature" and nice players around, they DO play the game and I have met some of them, not many I must say but that's a given, Diablo II is the type of game with a loot system that forces the most greedy natures of players out in the open and you'll realize that soon enough. You may even be one of them without being aware of it or without being able to admit it, as I've been myself during the first years... then I calmed down as I aged, I guess, until I actually quit the game, three quitting attempts later, roughly eight years after I started playing it.
So anyway, you've been warned. It IS a fabulous game, the game-play mechanics themselves work wonders, to me and despite some of its drawbacks the game is almost perfect, almost, since of course no games are actually perfect (not in my book anyway), but Diablo II is on my top three list of the very best video games that I myself have played in my whole life so far, it's just that good. Even though I haven't played it for even ten minutes since the past two years or so, or a bit less than that but I haven't touched it in a long time and I do not plan on going back at it. I'm waiting for Diablo III right now, and in the meantime I occasionally play Torchlight.
EDIT: Also, I highly recommend you to check out there basics of the game before plunging blindly in its world, check out the Arreat Summit, a well known and beloved database about everything you need to know:
http://classic.battle.net/diablo2exp/
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