The commish of my main league came up with this term. I asked the main players in my league what they considered the "fun killers". I had my idea in mind, but I didn't tell them what I was thinking because I didn't want to influence their answers. I'll post the responses I got here.

Up the average yards gained per play by 1 - 1.5
Decrease knockdowns, interceptions and pass interference by around 1/4
Increase sacks by 1/4
Create a stronger correlation between discipline and penalties
Minor tweaks are the way to go, we don't need a pendulum swing
The game is really really close to ideal as it is
Slightly smarter QB play could resolve a lot
I'd love to see some deliberate bootleg options in the mix
mobile QBs are the most fun to watch
Maybe decrease interceptions in man 2 man, increase them in zone

Someone else later seconded the call from running QBs, run-pass option. With the 0.4.5 passing nerf, that makes sense to me. I completely re-made my team into a run-first team, I kept thinking "Tom Osborne's Nebraska Cornhuskers, that's what I need..."

"Injuries. Less of them please."

follow up comment by another user:

"I'd love a run pass option
I don't mind injuries as a wild card factor, however I do have some players who seem more injury prone than others - if that is a stat, we should be told it exists
I don't need to see the stat, but I'll immediately start recording injuries myself"

I agree with this second point, is there a "prone to injury" stat? I'd like to know if there is. We don't need to see it, but if it's there, I'd like to know. Speaking of "seeing" hidden stats, or formerly hidden stats, Volatility should go back to being hidden, IMO.

What I think is the biggest issue is they out-of-control keying by defenses. I've faced opponents who keyed on every play, from the very first drive. Now, in the first half it was pretty random. They were often wrong, but sometime in the third quarter what I've been calling the Defensive Coordinator's "psychic powers" clicked in and they were keying correctly on every play. I had a 2nd & 10 run and it was run-keyed. That's just ridiculous, and for me that is the biggest current issue of the game. There are others, but if you were to ask me the one thing to fix in the next revision, I'd change that.

I've asked a long time ago, JDB said "no".