Re #1 - Ideally, I'd like to see parental controls set up as subaccounts where I, as the parent, can create the accounts of my children and set things like their ESRB level, specific game whitelists, or filter by game criteria (such as disallow games that have online chat components for example).1.) Do you mean requiring like a password to play Mature games on a single account, or just limit a separate account from downloading anything like that entirely? Either way should be easy for them to add. I wonder why they haven't.
2.) This will never ever happen. It's against all licensing rules as they're currently set up. Back in the day you had one "play" disc, and unless you copied that disc you could only play on one machine at a time. Steam works exactly the same way. Whoever has the "play disc" (or steam account now) has the ability to play the game. Steam works very similarly to how PC games have worked for quite a while unless you were doing something illegal (no-cd crack, copy disc, etc).
3.) Rumors are that they're thinking about trying this with Valve games. Obviously the other devs would have to make their own call if they want to join in. The recently added marketplace to trade/sell TF2 stuff some believe to be a trial for doing that with games themselves.
4.) Subaccounts would make #1 easier for sure. I could see them maybe allowing multiple profiles under one master account, but I think they'd still limit you to only being logged in to one machine at a time. Even the consoles don't allow you to log in with your account in more than one spot.
Re #2 - Again, with subaccounts, there's a viable and feasible solution right there. If any subaccount within a master account's umbrella is currently utilizing a given title, then no other subaccount within that realm can also use that title. Obviously there are some technical ramifications that would need to be resolved (such as what does this do to "offline mode"; what other caveats are there, etc.). The only technical limitation is that nobody has had the balls to try to innovate this sector for the benefit of the consumer because - well, why bother? It only serves to diminish a potential revenue stream.
Re #3 - That rumor has been around since forever and a day. Until it happens, I put no faith in it ever happening. Even when it does happen I simply don't see publishers ever jumping on board.
Re #4 - Again, there are plenty of elegant technical solutions to make this work, but just like consoles, the mantra is "sell sell sell", so I doubt it would ever happen. After all, why is the console industry so angry with the used game market, for example? It's all about revenue streams. Despite what some others have tried to subtly claim, not a single concept I'm pitching even remotely constitutes any form of piracy and is well within a consumer's typical legal rights. It's simply that we've gotten to a digital age where technology has managed to prevent us from fulfilling those rights - for the sake of profits.