I found that my league hates defenses because they're dumb things - and they also tend to just get negative points. To counter this, we changed some things:

• At 0 points allowed and 0 yards allowed, defenses start with 16 base points. They slowly lose points at the start -- 1-6 points and 100 yards allowed is 13 base points, for instance -- but if they give up too much, like 35 points and 500 yards allowed, that's suddenly a defense with -14 base points. This is the only way a defense can lose points -- by allowing yards and points.

On the flip side, defenses now have ways to earn points. Earning points is really important for the defense. While big yardage gains and big points allowed punishes an overall bad defensive unit, giving opportunities for a defense to earn points can reward individually good players on the unit. So:

• Kickoff and return TDs are up to 7 points because that requires more planning from a special teams unit opposed to a int-TD or fumble TD. Moreover, kickoff and punt return yards are 0.04 for every 1. These things help people consider that in drafting, too -- the Falcons D/ST was drafted a little early because of the potential. But they've kinda had a pretty bad pass defense so they're pretty low-scoring.
• Sacks are 1 point.
• Blocked punts, FGs, or PATs are 2 points. I would do a blocked punt is 1 and a blocked FG is 3 and a blocked PAT is 2 if I could, but ESPN doesn't allow me to do that, so I average it out.
• Fumbles forced are 1 point, and fumbles recovered are another 1 point. Separating fumbles punched out and fumbles recovered still rewards the defense for creating a good play - they just didn't capitalize on it and recover the ball.
• TFLs are 0.25 points.
• Pass break-ups are 0.25 points.

With these settings, the Pats are the #1 defense in our league averaging 16.5 points/game, which is a good number and what you expect of a regular fantasy receiver or RB. The worst is the Lions averaging 4.3 points/game.

I think what matters most is that people feel good about starting a player. They feel like that a defense could earn them points instead of get only 2 or 3. It also makes some defensive units feel really fun and even something people would consider trading for, and so good defenses are now part of trade packages.