The main fifference you will notice is in the shoes. Good road shoes are designed not to flex at all. Even the stiffest MTB shoes are designed with some flex so that you can walk in them. That will make a huge difference to how well you can transfer the power.
Buy the stiffest shoes that you can. To some extent this eliminates the need for a large pedal platform, as it eliminates any dead spot/numbness inducing pain from the pedal under your foot. Look at the Speedplay pedals that are so popular for example. I wouldn't personally consider buying them, but I only mention them as an example of a particularly small pedals that would be impossibly uncomfortable if you didn't have stiff shoes.
Having said that, a larger pedalling platform does have its advantages. You can't go wrong with Look or Time if you want float for your knees, but even the latest Shimano road pedals have quite a decent platform.
Don't worry about walking. As I said, you aren't really meant to walk any great distances in road shoes anyway, but most modern cleats have some sort of softer rubber 'outriggers' that are more grippy than the slippery cleat, and spaced wider so you aren't balancing on a small cleat.
And don't worry about clipping in and out of single sided pedals. The pedals are weighted so that as you turn the cranks, they 'present themselves' cleat up. You won't need to look down after a short while.
If you are serious about enjoying riding the road bike, definitely take the plunge. You'll realise how inefficient your mtb shoes and pedals really are for pedalling. They are inevitably designed as more of a compromise between a number of requirements (including walking), whereas the road shoes and pedals are designed with one purpose in mind. It's the old question of horses for courses. Road pedals and shoes for the road, mtb pedals and shoes off road.
The best advice would be to spend a bit less perhaps on the pedals and shoes, and spend a bit more on some footbeds. These make more difference than saving a few grammes on a pedal upgrade, or a flashier shoe.
And, resist the temptation to over-tighten your shoes. You would be surprised how much your feet heat up and swell. If your shoes are too tight, this can cause discomfort, and people often mistake this for issues with the stiffness of the shoes sole. When you first set out in the morning, you need your shoes much tighter than you will later on in the ride. Watch the pros. You will often see them tightening up their shoes towards a sprint finish. This absolutely helps with power transfer, but would be uncomfortable for the 200+ kms that they have ridden prior. You don't want your heal slipping out, but resist the temptation (which is easy to do with all the ratchest and gadgets that modern shoes have!) to over-tighten.
I clearly read that in typical STW fashion.
