Spurred on by TAFKASTR's thread, which "improvements" to bikes are genuinely worthwhile, and which are just "different" - so we can all keep spending £££ upgrading, when manufacturers don't have any genuinely new ideas that are worth buying?
My view: 31.8mm bars, 8 & 9 speed, 15mm, hollow chainrings on Dura Ace etc (FFS!), the numerous variations on the basic suspension concepts and no doubt many, many others.
I'd say the stand-out, genuine steps forward, worth the £££, are: indexing, hyperglide, suspension, spd, discs brakes, camelbaks, tubeless. (no doubt I'll have missed a few!)