I think the biggiest difference is not of technical but legal issue.
Frequency of 2.4 GHz belongs to so called 'ISM' bands ,more at :
while for usage of frequency of 5 GHz You usually should have a licence to use it.
While using 2.4 GHz,You have no guarantee of any kind of link quality because everybody can put amplifier and 'kill' Your 2.4 GHz link.
If using 5 GHz frequency and You have a licence,You can sue anybody who might be using Your channel.
5Ghz, is the ISM band, what you need a license for is the 4.9, which is public safety.
the biggest difference when related to 11n, is that in the 5Ghz, you can do channel bonding, and get a 40Mhz channel, and you still have non-overlapping channels. in 2.4Ghz, if you do channel bonding, you will get overlapp, and cochannel interferrance.
In terms of 2.4Ghz and 5Ghz atleast in most of the world both are unlicensed radio frequencies, and there are alot of differences in regards to them, aside from the 802.11n protocol.
In terms of the united states the 2.4Ghz band has roughly 14 channels, of which the U.S. allows you to use Channels 1 thru 11. each channel is 22Mhz wide and all frequencies are based on center frequency; the downside to this is that of those 11 channels only 3 are non-overlapping - 1,6,11 so you really only have 3 channels with which to configure devices; although some schools of thought will tell you that you can use 4 channels - 1,4,8,11. Globally it's accepted that you use only the 3 channels - 1,6,11. To use one of the other channels can cause impact to the others around it.
Now in terms of the united states the 5Ghz you have roughly 12 channels 36, 40, 44, 48, 52, 56, 60, 64, 149, 153, 157, 161 (although a recent revision has added 100, 104, 108, 112, 116, 136, 140, 165 - not all client adapters support these channels). Unlike 2.4Ghz all of the 5Ghz channels are non-overlapping, so you can actually utilize all the channels.
aside from just the channel issue there is another major difference between them, the 2.4Ghz band is heavily saturated around the world by non-wifi devices such as microwaves, cordless telephones, audio/video senders, security cameras, toys, bluetooth, specialized medical devices, etc. So when implementing a wireless network in the 2.4Ghz range there are a great number of interferring devices to contend with, the 5Ghz frquency is pretty much empty.
Here is a link that covers more about this stuff.
Specific to 802.11n
As mentioned above channel bonding is the difference. In order to take full advantage of 802.11n (300 mps) it REQUIRES 40 MHZ of space.
Think of a loaf of bread for a moment. You have 80 slices of bread in this imaginary loaf. Each slice is a piece of frequency and it starts from 2.400 - 2.479 ( i may be off a few -- going off memory)
Today you need 22 Mhz of spacing between channels, this is why you have 1, 6 and 11 as the main channels..
So channel 1 uses 22 slices from the loaf, channel 6 uses 22 slices from the loaf and channel 11 uses 22 slices from the loaf.
So now think if you deployed 802.11n on 2.4 GHZ at 40 Mhz spacing. You would use 40 slices of bread for one channel. This leaves just another 40 left for one more channel. So you could only design 802.11n on 2.4GHz with a 2 channel design, which doesnt work well.
On the 5 GHz side there is more frequency avaiable , or loafs of bread in my example. For example, you have channels 36 and 40. you can 'bond' both of these channels together to get your 40 MHz loaf to use 802.11n at 300 mps.
no problem... we will make a wireless engineer of you yet !