The only differences I know is that road bikes are often lighter and uses different set of brakes. But what else are the main differences?
Cyclocross bikes will have a slightly different geometry. More like a relaxed road geometry with a higher bottom bracket than a road bike typically. You'll be stretched out a bit less and more upright. Cross frames also will have wider tire clearances for 32mm to 35mm tires, but it varies by model as some can fit 38mm to 45mm tires.
Traditionally, brakes tend to be wide profile cantilever brakes for good tire clearance and less chance of clogging with mud on cyclocross bikes. However, disc brakes are now readily available on many cyclocross bikes. Road / Racing bikes tend to use caliper brakes which have better modulation, but typically have small tire clearances, which may be too restrictive for cross tires, especially when they are covered in mud.
The new standard rear wheel size for disc cross bikes seems to be 135mm, so it's the same as mountain bike disc hubs. Non disc cyclocross bikes tend to have rear hubs which are spaced at 130mm, the current road standard hub spacing.
Tires on a cross bike are going to be different as well. Road bikes will typically have skinny 23mm to 28mm slicks. On a cyclocross bike, you will typically have knobbier tires which may not last as long and provide more rolling resistance when compared to road tires, which is a tradeoff for having better handling on dirt / grass / mud. You can always swap out tires to match whatever style of riding you prefer. Depending on how serious you are about racing, you can find both cyclocross and road tires available as glue on tubular tires. However, some people have been adapting mountain bike tubeless technology and mounting cross tires as tubeless to run lower pressures for better offroad handling. It is not common to use the mountain bike style tubeless setup on a road bike, and generally not desirable.
Road bikes and cyclocross bikes tend to use STI shifters and similar groupsets (chainrings / cassette / derailleur combinations), however sometimes people prefer to use bar-end shifters for cyclocross (or retroshift shifters which mount bar end shifters on brake levers). Additionally, some cyclocross bikes have an extra guard on the outer chainring to prevent injury when mounting / dismounting frequently. These guards are typically not as heavy duty as mountain bike bash guards.
Cable routing tends to be different on cyclocross bikes due to the need to shoulder a bike during the race, so under the top tube routing is typically avoided. Sometimes 3 cables on top of the top tube or internal cable routing strategies are used. Often times full housing will be used to help prevent cables from getting contaminated with mud as easily.
Cyclocross bikes typically come with:
- Wider/knobby tires.
- Wider seat stays (for the wider tires) but this also makes it easier to fit fenders.
- Compact Crankset such as 50/34
- Flat underside of the top tube - for carrying your bike on your shoulder.
- All cables to the rear along the top of top tube (or internal) - again for shouldering
As noted, the brakes on CX bikes are either Cantilever or Disc, mainly for the clearance of wider tires and mud.
I am seeing more road bikes with compact cranks.
Cyclocross bikes value a different set of priorities than road bikes. You want your cyclocross bike to have wide grippy tires so that you have good traction in all of that mud. You also want more clearance around your stays so that said mud doesn't build up too much there. You also want to have a wider wheel base and more clearance for easier maneuvering through the courses.
Road bikes aren't strictly lighter, a carbon fiber cx bike might easily be lighter than some aluminum road bikes. But yes, in general you want to lose as much weight as you can if you're going to be racing that road bike up all of those enormous hills.
With all of that maneuvering, another great feature on a lot of cross bikes are the cheater breaks
An important factor is geometry.
The difference is tire clearance and brake type, yes ... but the difference between a road bike frame and a Cyclocross frame is measured in many more millimeters than the ones printed on the tire sidewall.
For instance, the 58cm Crux is somewhere generally between a 56cm Roubaix and a 58cm Tarmac, seat tube length withstanding.
The aerodynamic drag of CX bikes is more than that of road bikes, especially because disc brakes are not optimized for aerodynamic efficiency, as they are still largely just iterations of mountain bike systems.
As far as riding a CX bike in stock trim on the road, I think most riders would notice the 2cm average less drop on a CX's handlebars more than just about anything else (assuming the rider equipped the CX bike with 25-28mm tires).