What are the main things I should be aware of when buying one that are fundamentaly different from a moutain bike ?
The main thing to be aware is that road bikers are obsessed with not only light weight like mountain bikers are, but also about aerodynamics.
Thus, you will find stupid "aerodynamic" wheels with less than 36 spokes in many road bikes. Needless to say, you should stay away from such stupid wheels. Most reasonable-spoked (36-spoke per wheel) bikes are not marketed as "road bikes" but rather as "gravel bikes" or similar.
You will also find that the stock tires are very likely extremely unreasonable 23mm ones, despite the fact that 23mm tires give worse rolling resistance than say 28mm or 32mm tires. This is because of the fact that many racing cyclists ride at such high speeds that the most important consideration is not rolling resistance but rather aerodynamics. Many road bikes cannot be fitted with >23mm tires. If you're lucky, 25mm might fit both front and rear. If you're very lucky, 28mm might fit both front and rear but then you will not have any reasonable tire clearance for mud etc. For non-racing speeds, 28mm and 32mm are far better than 23mm and 25mm.
You will also find that the handlebars cannot be raised to a reasonable height with the stock stem, unless you purchase a needlessly large bike, and if you purchase a needlessly large bike, the handlebars can perhaps be moved to a reasonable height but are too far away from the saddle and bottom bracket.
The obsession with light weight has gone so far that many road bike components are not durable despite the fact that with high-pressure narrow tires and no suspension, the components receive far higher loads than mountain bike components do. Thus, plenty of them fail early and often. This is exacerbated by over-using foolish materials like carbon fiber.
The chainstays are often very short, so if you're tall and pedal uphills sitting on the saddle, the front wheel rises from the ground. Such short chainstays are marketed as "light weight" and "quick handling". Reasonable chainstay lengths cannot be found usually on bikes called "road bikes".
You will also find that road bikes have wobbly brake levers, because of many buyers with $$$ feeling a need to be able to shift without moving hands. Thus, reasonable shift levers like bar-end shifters have gone nearly extinct. It will take some amount of time to get used to such wobbly brake levers, if coming from a mountain bike background where such wobbly brake levers are not used.
You will find that tire pressures are far higher. In fact with 23mm and 25mm tires you need so much pressure that any mini pump with reasonable design takes hundreds of strokes to fill the tire. A mini pump with larger stroke volume would require unacceptably large hand force. My solution to this is Quickex Quicker Pro pump that pumps on both in and outstrokes, but you can't find such pumps for sale anymore because the CO2 filling systems killed their market share.
The chainwheels are larger, but then again on road that's a necessity. On road bikes, the small chainwheel is typically the same size as the large chainwheel on MTBs -- too small for anything except uphills.
Many "road" clipless pedal attachment systems do not allow walking on the shoes, so stopping at a grocery store to buy something to drink is out of the question -- but then again, if you afford $4000 carbon fiber road bike you obviously cannot afford $2 drink at a grocery store and have to carry a water bottle with you (fortunately, nobody weighs water bottles so its added weight does not matter). Thus, you'll want to stay with MTB-style SPD systems even on a road bike.
Don't buy a road bike. Buy a reasonable bike. Some time ago, such reasonable bikes were most easily found by the name "touring bike" (although such bikes often had too low bottom bracket), and nearly reasonable bikes could be found by the name "cyclocross bike" (although such bikes often had too low handlebars). Today, the hot word for reasonable bikes seems to be "gravel bike".