Depends how far back we’re going, I’ve been doing this for a long time, so:
Tyres. They are so much better now than 10 years ago.
Flat pedals. They used to be horrible cage things, they’re now grippy, lighter and just better.
Disk brakes. They work, they’re reliable and they make MTB’s and road bikes better.
Di2. A proper indulgence but it’s so good to use.

Pretty much everything that’s been said but for me the biggest one that is always memorable is full hydraulic disk brakes.

I remember when V brakes came out they were awesome but they were still quite poor when the weather was foul and required setting up perfectly. The parallel arm things on XT+ were good but became sloppy very quickly.

When I first put a Hope C2 brake on the front of my bike it was a revelation. Much more braking force in all weathers and a genuine amazement for 6 months or so until I got a frame that could accept a rear brake, where I soon became used to such power.

Tubeless is up there although not easily thought of as a game changer because not getting punctures doesn’t really make the riding better. Not sure I can feel much difference in riding as I am too fat to use really low pressures.

Di2 on road bikes is immense and I’d have it instantly if I had a desire for tarmac.

Hydro disk brakes and indexed gearing have been the best improvement for me

Tyres. They are so much better now than 10 years ago.

I’d love to see a back to back test of a mid 80s Specialized Ground Control and the modern equivalent…

Most things have improved really.

If you look at my current bike a Ramin 3+ it looks superficially very similar to my first mountain bike a 1986 Rockhopper, even the colour is not that different. To ride though there is absolutely no comparison. Every single thing on the new bike is better and nothing on it requires anymore maintenance or faff, it’s still a simple rigid bike. The Rockhopper cost £400 which according to a quick online UK inflation calculator would equate to £1141 when I bought it last year. I think I actually paid less than £750 for it. Even when they were at their highest price they were £900 and regularly discounted to £800. So not is only everything better than it was all the improvements cost less.

I remember when V brakes came out they were awesome

They certainly stopped you better than cantilevers but I also found they jammed up more readily in mud. My off road commuter has them on the back and in winter I can be stopping regularly as the rear wheel stops going round if the mud is at the wrong consistency

Anyone else had the rear U brake under the seat stays? That was not a step forward!

Yeah i did, it worked fine tbh.
Cantis were a step back in terms of power, and had worse modulation too.
But **** me, they weighed a bloomin ton!!

One of my early bikes was a 1991 Gt timberline that had a rear U brake.

Didn’t really notice anything about it although the bike was made from iron and incredibly heavy.

Gave it to a young relative when I got a Pantera in 1995. Such a different bike, especially with the ‘upgrade’ of some Quadra 21 forks.

Everything above, plus LED lights. Making winter night spins less of a faff.

Electric bikes.

… not really. Specifically I’d say: disc brakes, decent flat pedals, indexed gears.
More generally I’d say the kick up the arse that ATBs gave to the entire bike industry.

Forgot Aheadsets. Total game changer. Most of you will have forgotten the trauma of changing stems and fitting bikes. It was just pain. With an aheadset and modern stems, the number of frame sizes has reduced and the ease of swapping and fitting increased immensely.

Nothing wrong with traditional headsets, but aheadsets with preload are just so much better.

Anything that requires massive spanners to adjust and regularly comes loose has plenty wrong with it!

They were ok on road bikes but headsets were a pain for off road use.

I’d go with geometry. At 6’3, old school 26″ bikes never really felt like they fitted properly. Nice long modern 29ers do though. I could live without a few other modern features in exchange for a bike that fits/handles nicely.

Multitools.

Did anyone else have those terrible “spanners” stamped out of mild steel that had most of the sizes you need to keep your bike going?

Invariably they would bend, and/or round off the thing you were working on.

I remember going into a bike shop a lifetime ago to buy a new one and they had stopped stocking them a decade earlier.

the original v8 and v12 pedals.

So many small improvements have created mega machines.
Indexed multi shifts up and down
Remote Droppers
Shift away from 1 wheel size
Suspension
Disc Brakes
tubeless
Bikebuilders CHOICE
Trail centres
Carbon

When you think of all the changes over the last 30 years I don’t think there’s ever been a big shift, everything is just a small improvement from what there was.

Disc brakes, tyres, air shocks.

Recently I’ve swapped out my air shock for a slightly reduced travel coil shock,and it’s caused an astounding improvement in the bike for me.

In approximate chronological order:

Indexed Gears

A-head Sets

Disc Brakes

Descent Suspension

Tubeless Wider Tyres

Dropper Posts

Improved Geometry

SPDs for me. Prior to that is was clips and straps, or power grips.  Cross racing tells me i can go back to rigid, small tyres, and v brakes but I’d never swap out the SPDs.

Oh yeah, Id forgotten led lights, at last batteries lasted more than one ride!!
When i used to ride to hamsterley forest at night, i took 4 sets of rechargeable batteries with me, to power a halogen vistalite, with an absolutely puny 2.4w bulb.
A low powered modern led light would blow that away.

In order of adoption for me:-
V-brakes
Headsets
Index gears
SPDs
Suspension fork
Disc brakes
Lock on grips
Rear suspension
non-square cranksets
Speed link
Dropper posts
LED lights
and more recently that wee button on Sram rear mechs for removing the wheel.