Its funny that you put it that way. I agree that old school curvilinear left us around 2000, but I think it's spirit is still around in a lot of corps. Writers like Pete Weber, John Vanderkolf, Jamie Thompson, Jeff Sacktig, and to an extent Myron Rosander(because its apparently the only way he can write) still do a darn good job of integrating curvilinear forms and progressions in with more modern stuff. (Additionally, Michael Gaines wrote some incredible large form curvilinear drill, we just don't remember those moments) IMHO, BD is one of the biggest killers of curvilinear drill today (I can't include SCV2010 in that because they didn't seem to have any discernible style). BD used to have some gorgeous curvaceous drill and now they are doing this thing where they might do 2-5 sets curvilinear and the fall to a scatter, just as a previous poster noted.
Also, modern drill actually is harder to march, not just looks harder. Because of the very exposed lines of the 80's & 90's, that drill was much harder to clean, but, frankly, I think kids with modern training could make easy work of it.
Back to the subject at hand. I agree with several of the previous posters that the switch to any key brass has been a huge advantage to the activity. Corps can recycle lines more often and they are getting a much better product to boot. I've played G bugles, good ones too. They are clunky and out of tune. Yes, the larger bore size, additional metal, and longer bore allowed them to project more...or at least allowed the player to blow harder before exceeding what the horn could handle. At first, you could really notice the difference in volume between lines, but now as manufacturers are making better horns (they get to use DCI as a test laboratory) and the corps figuring out how to play louder, I think the volume level is equaling out. (IMO, Crown2010 is the loudest horn line I've ever heard, and with the best overall sound too!)
Recruitment restrictions. Love the age thing. This has helped everyone. Switch to 138 was a good idea. I don't know if any corps have picked up an extra bus for the switch to 150. I'm not entirely sold on its virtues given the loss of so many smaller corps. Time will tell on that one. I think it may be cyclical (as regional corps die out, there will be a short period with very few and then the absence will show the need for more small corps.
Amplification...well... In 2004, I was one of those anti-amp people (yes, I have a t-shirt). The learning curve on that one has been pretty long, and I don't really think everyone has figured it out yet, though we are getting closer. Some great victories for amps include the weird drum from BD2004 (timpanist played it & we wouldn't have heard it otherwise), Cajons from Madison2008, and an ever increasing group of corps that are figuring out that the amplified sound of mallets should REINFORCE their acoustic sound, not cover it up. An issue that is exacerbating any problems is the sound guys. I know that not all of the board workers have a background in live sound, but they almost all behave like it. WAAAAY too loud. Its like going to an outdoor symphony concert with a sound guy that used to tour with guns 'n roses. I don't know if anybody has done it, but having a guy in the stands w/a walkie would really help things out.
Amplification will turn out for the good in the long run...I think the reinforced mallet and pp instrument sounds are a boon to the activity. It may just be a few more years until everyone gets it.
Amplification of voice. It can be used well, such as Crossmen2006. It can also be brash and ugly. I think we've seen the extent of its use and will only see limited use in the future.
Electronics. Open can of worms: I think it will turn out to be a great thing. Right now so many design teams are still figuring it out that its not always a positive addition. For instance: so many corps use CRAPPY piano patches. Best one of the field was Crown (also had a great guitar patch). I think their use of piano and guitar added to the production in a positive way, which is what everyone should be aiming for. Madison's piano moment (rhapsody b4 drum break) was simple and short. It reminded us that they're playing a piano feature while not smacking us in the face with it. Unfortunately, their patch was not that great (very tinny). I agree with many that the thunderous goo is bad. It has its place, but ideally we shouldn't be able to hear only that during a tuba moment (spirit). Also, STOP SYNTHING SOUNDS THAT CAN BE CREATED ACOUSTICALLY!!!!!!! That is just lazy. Designers are showing their creative bankruptcy with that. I don't want to hear a synthesized brake drum sound when you have one on a rack. Yes, you can make it louder with a synth..that is what amplification is for.
Design. Everything is getting more theatrical. YAAAAAAAY. I love the good old shows from the 80's and 90's where you can just sit back and enjoy, but so many shows today now offer an additional level that can be more intellectually rewarding (the sit back shows are viscerally rewarding). Only trick here that several corps have missed on is that the rest of the show has to be able to stand without the theatrics. They must only add to a great program, not be the cornerstone of it.
I have more thoughts, but have to run.