This is an aspect of many RPGs I’ve played, and I think it would greatly enhance the RPG skills aspect of Enemy Territory and encourage players to make teams well-balanced in regards to skill and XP. This in turn would make the game more fun for everyone involved, as the games would be more competitive
Players should gain more XP for killing higher ranking players, less for killing lower ranks. So if you’re a General, you’d score almost no XP at all for killing privates. But if you’re a private and you kill a General, you score even more XP.
In RPGs, this system is implemented so that high level characters are strongly encouraged to challenge themselves to progress at a faster rate by fighting monsters near their level, instead of merely butchering the easiest monsters around.
Right now, ET encourages people to achieve higher ranks and higher skills by helping your team, and all enemies you kill are treated the same when it comes to XP gain. A problem arises when one team consists of much better players, or people with more XP/skills, so that it totally dominates the other team. XP shuffle votes can be called, but usually fail. Why? Because the dominant team wants to get XP, wants to get skill, and wants to get higher ranks. And fighting a totally outmatched team of newbies is the easiest way to do this.
But, if player rank was taken into account in regards to XP, then people would naturally want to balance the teams by rank on their own accord, or use the XP shuffle feature more often. This way, to speed up their own climb up the ladder of power, they’d have to kill people closer to their higher ranks.
Another great feature of this would be that people might specifically hunt down certain high rank targets, which is pretty realistic… not to mention FUN! 
I should also add that you shouldn’t be punished too severely for killing people who are merely lower rank than you, but who are MUCH lower rank than you. The farther away they are from your rank, the higher/lower amount of XP you’re getting. Just like in RPG games. Just to clarify, folks!
Crazy monkies.