- Steeper skirts (the ratio of bandwidth at -60dB to the bandwidth at -6dB). 1:1 is ideal but 1.5:1 is about as good as they get.
- Greater ultimate rejection of signals outside the passband. IMO, better antennas require better ultimate rejection.
- Less passband ripple or flatter passband "frequency response".
Since it's after the front end, an IF filter can't make the front end quieter. However, a narrower and/or sharper bandpass results in lower noise power delivered to the detector. On some signals, I can hear them much better when switching to a narrower filter. A narrower/sharper filter also lets less QRM through to act on the receiver's AGC. To a point, it's silly to use an IF filter that's wider than the signal you're trying to hear (very narrow filters tend to ring-g-g-g).