In Spiral Knights, there weren't many parts of co-operative play, because most of the time people get in each other's way more often than actually benefiting from each other.
I first saw this game at mmohut.com and at that time games had more emphasis on fun and less emphasis on taking things away at the cost of fun for the sake of this or that. It was actually very worth being on the lookout for new games.
https://youtu.be/mUEv7unHQKI?t=521
Here if you skip to 8:40 you can see an example of how co-operative play already fails to work, although by seeing the contraptions you get the impression, that it would be somewhat co-operative. It still makes a funny experience, i admit lol.
Well anyway, most of the time people either knock monsters out of each other's way or directly into each other during the monster's attack animation, or people don't get to attack something, because it was already killed by a team-member. That last part is actually not as significant, because there are usually enough monsters around for everyone.
In any case, one of the major factors in favour of teamplay were health-share revives. Health-share revives from my perspective made it possible to set a challenge to yourself. One benefit was, that you could focus on reviving somebody first, so in the case of a total wipeout, 1 person would become a source of hope. Another benefit was, that no matter how good you are, you can always get hit and reviving someone not only added some safety and a "challenge" to it, but it's always a good deed. Alot of the time nowadays a form of "assisting" somebody in a game is not an actual help. You are taking away or literally stealing their gameplay. But with health-share revive you were giving them gameplay. Sure, sometimes the pressure was too big for some people, but you could afford to be optimistic and they would always be welcome on the battle field. Nowadays you get the 1-of-2 situation where you are either the dead-weight or the elitist. It works a portion of the time, but the team spirit can never be the same. And if you run out of life, it just comes down to money..or "sparks of life". I don't mean to suggest bringing back the constantly increasing energy cost system, which would still be nice, if it wasn't tied to in-game money, let alone real money. But the idea of spending something to revive, especially something shiny, does not resonate with me very well.
And in the context of the health-share revives, it also makes no sense to me, why certain monsters were adjusted or "fixed". When zombies or retrodes kept aiming at a player instead of blindly attacking in 1 direction, it may have been difficult, but it also added a layer of challenge to the most-commonly-visited stages, making it actually reasonable to even bother thinking about a revive system.
The most endgame stages, that i have tryed, were so difficult, that i can't even think of beating them with all the sparks, that i can possibly save up. I don't think, that the health-share revives would make a difference for those kind of places..
Similarly, the droppable items like health pills, remedy pills, and vials were an essential benefit to party play !
And to me, these weren't some sort of fine-tuned mechanics, no..i viewed them as the beginning of the road, as an introduction, where i can hope to expect future improvements. But instead people viewed them as a flaw..and "fixed" them.
If I understand correctly...
Your main objection is that health-share reviving was an important kind of teamwork. I agree. There were many complaints when this feature was removed.
Your second objection is that some of the AI has been weakened. I tend to agree, although it's not all bad. For example, wolvers no longer track their target while they attack, but instead we have gorgos with approximately that style of attack.
Your third objection is that knights should be able to drop pickups, so that other knights can pick them up? I think that this is tightly tied to how treasure is allocated now. Everything is individual to the knight. I mean, I agree with you, but this change would require significant changes to the code, which would have other effects on game play, some of which you might not foresee.