I think a lot of people throw a lot of red herrings into this discussion. I'll try to be concise...
1. Suspension.
Now that PD valves are cheap and easy to get, the one definite advantage that pre-90s had (cartridge forks) has mostly disappeared. PDV equipped evo bikes will be very nearly as good as the best pre-90 forks. Remember that the early USDs were all pretty shit.
At the rear, modern shocks/internals will bring an Evo bike close to the level of the best pre-90s. The amount of rising rate that's built into a 2010 rear end is only marginally more than an Evo bike with laid-foward shocks - and its tiny. As Tom68 said elsewhere, the bump-stop provides far more progression! The real advantage of pre-85/pre-90 style linkages is weight placement.
2. Brake wise, a pre-90 will have more-or-less the same front brakes as a pre-85 bike. In turn, a clean, properly set-up TLS drum is nearly as good as a pre-90 disc - the disc's real advantages are in the mud and that it requires less maintenance.
Rear discs are the same story - on a properly maintained MX bike, they offer a very marginal improvement in lap times.
3. Engines. Pre-85 is a big leap, compared to Evo - water-cooling, better factory ignitions and exhaust power valves make for a clear technological leap. From Pre-85 to pre-90, the steps are smaller - better power valves, more widespread use of plated cylinders, and the usual incremental changes.
Compared to a modern 2-stroke, the pre-90s are still lacking things like long stroke motors, 3D ignitions, solenoid controlled power jets, and another 20 years of incremental improvements.
It is also worth noting that a lot of new tech has already found its way onto (even) pre-70 bikes - so the stuff like good ignitions and modern pipe designs is irrelevant for the purpose of this discussion - you can assume that ALL serious VMX race bikes have it.
4. Other stuff. What is there?
Ergonomics were incrementally improved, but pre-90s have still got banana seats and low, rear mounted handlebars - they are long way a modern bike! Moderns run very flat seats (with rounded profiles), the bars a long way forward and typically very high. Moderns are also much narrower between the legs
(I originally typed "pre-90 bikes are fat between the legs" but I knew that Turtle wouldn't be able to resist...)Cosmetics. Again, steps foward(?) with fashion, but still a long way from anything like a modern.
Anyone who thinks that a pre-90 is 'just like a modern' hasn't ridden a modern. To seriously state that because a bike has dual discs, water cooling and USD forks that it must perform similarly to a new bike, is like saying that a XF Fairmont is just like a FG XR6...
By the same token, I'd love to see how much you'd have to pay Jay Marmont to race a 1989 YZ250 for the National MX title, no matter how much you tricked it up. If the 21 year old bike was even vaguely competitive, then I'm sure he and CDR would jump at the opportunity...
Now... the riders. This gets a bit harder to make straight-foward statements about, but I'll try.
a)
Everyone loves grass track MX, even young guys on moderns (look at Amcross, the old Thumper Nats, etc). There is absolutely no reason to assume that pre-90 riders will somehow be an exception to this rule.
b) The fast guys are the fast guys, no matter what you put them on. There's talk of pre-90 somehow resulting in an influx of lunatic 18 year old gun riders, but those same guys are either not interested in old bikes, or are already racing in VMX. If it turns out that I'm wrong, and we do get an influx of loonies, then at least they'll be in the pre-90 class where they won't bother the nay-sayers.
c) The guys who are keen on pre-90s because they are the bikes they have a connection with, are all in their late 30s or older. How old were most of the riders back in the early days of VMX (when it was only pre-75)? Did the older blokes stand around bitching about those 38 year old young lunatics then?
d) There was lots of talk about how Evo/pre-80 riders would demand rougher tracks, but it didn't happen. Then there was all of the same talk about pre-85 riders doing the same, but that didn't happen either. The greatest "crime" the Evo+ riders have committed, is being
tolerant of newer-style tracks - but modern clubs are building modern tracks for 40 weekends in a year, so there's no way that a few dozen Evo/pre-85 riders' voices would ever change that.
Some people like to talk about pre-90 riders as being somehow wildly different to the existing VMX riders. This is utter nonsense, as the guys who are making the noise about pre-90 are all established VMXers...
Personally, I'm a fat old wobbler with a mortgage, offspring, 9-5 job and all the rest - I don't understand how being allowed to race my pre-90 bike is suddenly going to transform me into a Southern Californian supercross legend?!
There's a ton of reasons why pre-90 deserves its place in the old dirt bike racing scene, but I'm going to concentrate on killing the red herrings for now.