I dont think any of these games besides Planetside really are Massively Multiplayer FPSs, but as for large scale FPSs, Battlefield 1942 running the Desert Combat mod is still the best multiplayer experience ive EVER had.
BF1942 and BF Vietnam are ok, but the Desert Combat mod for 1942 blows them both away by a lot. Mostly because of scale\realism. Tanks actually fire over a mile in DC, rather than 100 feet in BF1942 and BFV. I still play Desert Combat all the time, and it hasnt been updated since some time last year. Now I have become a master at anti-aircraft weaponry, especially on the Iraqi team on El Alamein
BTW, I agree with what you said about the M60, and you'll be happy to know that not only is DC fun, but it makes sense too. The United States side has an M249 SAW ... the Iraqis on the other hand have a PKM. More power, but it overheats faster.
EDIT: I also tried the Joint Ops demo and noticed the same things you said, plus it runs terrible compaired to other games that look 10x better. That game just all around sucks in my opinion. Feels like a budget title where they created a bunch of features, tied them together loosely, and called it a finished product. None of it really seems to work that well.