L4D is an amazing game, even in versus mode you're still killing a lot of AI zombies. And the only reason L4D would be considered "un-fun" would have to do with the players rage quitting after 1 round instead of playing the whole map. That and the match making was pretty terrible.
DTV is a fun mode, but you lose a ton of money playing there. I don't think they should buff it to the point where people will be here farming gold/xp, but they should buff it a little (maybe after each wave you get a tick?, keep it at x1 until you beat the 3 waves, and then go to x2 after each wave, after the next 3 waves you pass it goes to x3). Seems like an obvious tick/xp/gold system to me.
That being said, if you're farming gold/xp, go to the battle server and help influence your team to win. Sometimes you get in some bad streaks, but generally most nights that I play I have good periods of time where I'm rolling x4's and x5's for 30 minutes or more. But then again I'm a good influence on my team (not-so-humble brag).