I really don't think it would be that difficult, and I really don't think that a majority of people want to have a game that is built around partying either. You can have plenty of party-based events and still have decent soloing options, I just think some people don't want others to have a viable solo option, because you want to party, and when there is a viable solo option the amount of people actually willing to form parties goes down drastically.
I find it hard to believe game developers couldn't implement a system that allows people to solo mobs around their level for a decent amount of exp without it taking all day to kill a single mob, while at the same time allowing parties to fight mobs of higher level for an elevated amount of exp. I would say it's pretty condescending to think they are incapable of doing something so simple. The problem is, when they try to, people bitch that the game is too easy, and not enough people party, and it's a WoW clone, and blah, blah, blah, blah, blah.
There is no sense in acting like you cannot have a viable solo option, while at the same time having a party option that is somewhat better, and to say you think soloing should be slow and boring, is frankly you wanting to restrict other players based on your own damn preferences about leveling. This party or nothing attitude is bullshit and seems to me a sure way for this game to fail at PS3 launch, because I'm sorry, but many modern MMO players (perhaps the majority even) do not want to be drastically gimped by choosing to solo, and thus, coerced into partying in order to accomplish anything. Some of us have very hectic lives, work odd hours, live in various time zones, etc, etc, etc, and imagine this, some of us may not even like partying. *gasp*
Partying obviously has it's place, and should probably be required on the majority of end game content, but there is more to consider than this. I can agree, that people shouldn't be able to easily solo mobs 10+ levels higher than they are, however, I do think you should be able to solo mobs higher than you, IF you've put in the time and effort to acquire all the best gear, skills, etc. So, I'm not saying the previous system did not have faults in need of tweaking, but to go to the extreme opposite end of the spectrum is not any better.