You can see how Egyptian style has penetrated America in this clip. Some of the major style changes are the veil being entered with in the Egyptian style and worked with in the American. A major cut-back in cymbal playing since dancers didn't see them being played by Egyptians (due to the difficulty of synching them with video), and very little floorwork done anymore (which is partly due to changes in costume fashion, partly due to lack of venues with clean floors and good sight-lines from the audience to the stage, and partly due to it being banned in Egypt since the 50s). There has also been a few new props added, like fan veils, Isis Wings, and veil-poi. In many ways the Modern American style is a product of the vintage style being influenced by the vintage Egyptian.
Another situational shift that influenced the dance: during that boom in the 60's and 70's the number of bellydancers, both professional and hobby, swelled and created enough of a base of American fans that it wasn't just Arabic nightclubs and their American guests watching anymore. Troupes like the bellydance superstars showed up, basically the riverdance of bellydance, they are a fusion troupe originally directed by Jillina. about 4 minutes into this clip from the documentary American Bellydancer you can see Jillina stop Miles Copland from putting beginner dancers in front of an audience expecting pros (sorry I couldn't find a shorter clip). Before Jillina, Suhaila chews Miles out pretty good, her troupe is another that shows the American troupe style, of very precise technical dancing. Soulfire, in Portland, OR is another example of this troupe trend.
The internet has also allowed for dancers to reach enough of an audience to support projects that wouldn't have been as viable at local levels, for example, Cheeky Girls Productions created Project Bellydance, a bellydance reality show (without the cattiness, don't worry, it's a fun show ^_~). There are many other competitions out there (such as the one soulfire is at in the clip above) which, in addition to the shift from live to recorded music, has also been pushing the technicality of the modern American style.
The competition circuit creates a whole other world of hyper modern, ultra-technical dance dominated by the Eastern European dancers and technique that could keep a small nation of chiropractors in business. Dancers might fall into the trap of over-emoting in an attempt to prove to judges they know the meaning of their songs, and the purpose of performing can become distracted from bringing joy to the audience to winning something for the dancer's resume. HOWEVER many excellent dancers also make appearances at these competitions, and take the best of both worlds, like Oreet, or Shems. (I'm NOT including any links to the "what not to do" category. I want to make it explicit that all the links given are dancers I admire.)
Lastly, I'll just point out that these days we have access to any style, Lotus Niraja dances a style blending her own sassitude with Lebanese and Egyptian elements. Here's a dancer from Portland, OR who specializes in the Turkish and vintage American styles, performing to an Egyptian song with American veil work and Turkish style dancing, and she rocks it!