Multi-alt and 1-alt require different skillsets. On 1-alt you can watch over your character more closely so you can time attacks better, use a healing potion when you need to or even just log off or retreat (if you can) if you suspect you are not going to survive, but you do have to rely on your allies being kind and attentive, especially the person using the Cleric during boss fights, and passing them extra mana when they need it.
On multi-alt it is much, much harder to micromanage your party, especially during a major fight, where you have to pass mana to your Cleric when they run low, and some of the tougher bosses make simple macro-spamming very cost-prohibitive. For example, the Drider Warlord has a 10-point poison... while this is not much at the Grandmaster levels, it causes you to waste heals where you really need to use it to heal characters who have been hit for 100 damage (and the boss attacks 3 targets at once at a fairly rapid rate). Sleeping actually discovered it's one of the few times where Circle of Healing is useful. To give another example, monsters with high poison are tricky to deal with because it's awkward to check the whole party to see who is poisoned while you are in the middle of a firefight and casting Cleanse on them while healing everybody else.
The overall themes/requirements... multi-alt requires self-sufficiency, and 1-alt requires diplomacy (albeit a pretty twisted form of it these days).
In a way, multi-alt is easier because there is a lot more to do by yourself, but you have to put in quite a lot of investment first, which can take many months or years. Nevertheless, there are still opportunities for teamwork, like letting a lower-level or lesser-skilled player tag along with you on desert trips, and certain high-end bosses like Resca, the Stone Dragon and Woobie do require teamwork just because the damage output is so much higher than anything else in the game, whether it be multi-alt or 1-alt. Woobie, for example, can hit for 350 damage, has invisibility, has a 50-point poison, incredibly high dexterity and, to top it off, self-heals - the incentive is that she drops 50,000 gold and the Assassin's Tear, a 30bd dagger.
The increase in levels from 30 to 40 did tip the balance a bit. Tirantek and the Pyramid of the Mortal God were originally designed with teamwork in mind, and back in the early days, very few players could solo the areas (for a long time, Trevayne was the only player who could solo Imhotep), so everyone had to up their game, not just in crit count but also the spells used, spells like Consecrated Embrace and Almighty Retribution just so players could survive the opening volley from Imhotep and his two Mummified Priests. In an unfortunate way, Tirantek is what pushed players to use 20 crits, sometimes even more before the limit was put in.
Two good thing that come from the increased levels though... you could get to level 30 very quickly with the best equipment, with no real way of proceeding, while level 40 can take many years and certainly something to boast about. Secondly, at the Grandmaster levels, Dexterity modifiers don't have as much impact, so other stats like Strength and Wisdom, for example, become more appealing instead.
Finally, on multi-alt, you have to spread the costs over your entire party, so while you get more gold, you ultimately spend a lot more too, and buying a Cobalt Staff for each and every hitter is no easy feat. While the high vamparic rating makes things pretty easy, the effects don't have as much impact compared to 1-alt due to the macro-spam nature of Clerics, which is why the Sword of Existence and, later, the Soul Ripper and Twin Axes of Li Kwei, are appealing for Berserkers... greater damage output, while the reduced HP regain is already largely covered by Divine Restoration.
Other minor differences... on multi-alt, Constitution modifiers are practically useless because Thieves are almost never party leaders, while on 1-alt they can very easily make a Thief overly powerful. Similarly for Charisma modifiers on Rangers, since Hypnotise is seldom used.