di5tortion's writeup is interesting, but I have some comments on it, based on my experience as a DJ who spins progressive house (among other genres of electronic dance music), I have some comments on the issue. (Anybody feel free to disagree with me--I've only been at it for a couple years now, so I don't have as good a feel for the development of the various styles as, say, someone who's been into it longer.)
- di5tortion makes the comment that prog house is meant to be easier to mix than straight trance. I personally don't think that one is more difficult to mix; I do think though that your typical epic trance is somewhat boring to mix in comparison with prog house, because there's not as much room for the DJ to play around with the tracks; the melodic nature of epic trance makes it necessary for you to wait until almost the end of track to start mixing into the next song--either that, or go through write down what key all your records are in. This isn't to say that aren't fairly melodic prog house tracks, but there are also tracks that are a little more minimal and give the DJ more room in the mix--it's always fun when I can hold two tracks together in the mix for three or four minutes and have it sound good. :) I like musical interest as well, though, so I try to strike a balance between the two in my sets.
- The main difference I see between straight trance and progressive house is that trance typically has more of a straight, driving beat, whereas prog house normally has more complex percussion and has more of a groove to it. As for melody or lack thereof, "progressive house" is a constantly moving target so some of it leans in more of a melodic trance direction, but there's also some overlap with tech house or deep house.
- The attitude among DJs of wanting to have the newest/most obscure tracks to play makes the market more accessible to independent producers and small startup labels. This combined with the fact that the trance and prog house genres have become fairly well-known (and widely played) makes it so that there's a lot of junk at the record stores you have to sort through to find the good tracks.
- I didn't know there was a specific style called "dance" music. One of the general characteristics of most styles of music that become even remotely popular is they are at least somewhat danceable, so I would consider the term "dance music" to be awfully vague, since most rock, pop, and hip-hop could techically be considered "dance music".