In terms of 'good / bad', it's basically a trade-off between bandwidth and distance the signal can travel. A high carrier frequency allows a higher bandwidth[1], but it doesn't travel far/well. A low frequency can travel far, but doesn't support high bandwidth. So there is a sweet-spot (which is currently considered 'good') that achieves a balance of BW and distance. There's a great guide here:
1] the carrier frequency doesn't itself determine the bandwidth, it's the frequency range that does. For example, if someone says "a 100 Mhz signal", that doesn't tell you what the bandwidth is. You need to also know that the signal is actually allowed to cover 99.9 Mhz to 100.1 Mhz, so it's bandwidth is 0.2 Mhz (which could transfer ~0.2Mbps (order of magnitude accurate), depending on the encoding system used[2]). But if someone says "a 100 Mhz signal" you can guess that they don't mean the signal is allowed to cover 70 Mhz to 130 Mhz (60 Mhz of bandwidth), because: a) that's a ~2x range in frequency, which make it hard to design appropriate aerials, filters, modulators, etc; b) that isn't how spectrum is actually allocated anywhere I know of. So in practice you can normally deduce the approximate bandwidth by knowing the carrier frequency. Apologies for the long explanation - I've been nitpicked before on this..
http://www.bbc.co.uk/rd/pubs/spectrum/bbc-the-spectrum-and-i... (see page 5)
1] the carrier frequency doesn't itself determine the bandwidth, it's the frequency range that does. For example, if someone says "a 100 Mhz signal", that doesn't tell you what the bandwidth is. You need to also know that the signal is actually allowed to cover 99.9 Mhz to 100.1 Mhz, so it's bandwidth is 0.2 Mhz (which could transfer ~0.2Mbps (order of magnitude accurate), depending on the encoding system used[2]). But if someone says "a 100 Mhz signal" you can guess that they don't mean the signal is allowed to cover 70 Mhz to 130 Mhz (60 Mhz of bandwidth), because: a) that's a ~2x range in frequency, which make it hard to design appropriate aerials, filters, modulators, etc; b) that isn't how spectrum is actually allocated anywhere I know of. So in practice you can normally deduce the approximate bandwidth by knowing the carrier frequency. Apologies for the long explanation - I've been nitpicked before on this..
2] see: http://en.wikipedia.org/wiki/Nyquist_rate, http://en.wikipedia.org/wiki/Bit_rate