A dry suit will need an inflator hose. You will have to make sure that it is compatible with your regulator set up. Usually not a problem though.
Liners come in a variety of types. Just make sure you dont try to skimp and use sweats or some other type of non-diving protection. It didnt work for me.
You will want to make sure your hood is compatible with your dry suit. Cold water hoods come in a variety of styles with the neck cover being long, short, different cuts etc. Dont get a hood that is too long and bunches up around your neck. Dont get a hood that is too short and when you turn your neck you end up with exposed skin.
Gloves for a dry suit come in many styles. The main differences are wet gloves and dry gloves. The wet gloves come in various thicknesses with 5 fingers, 3 & 2 fingers and mitts. The dry gloves have a ring that attaches to your suit and the gloves mount to the ring. Some of the dry gloves have a seal inside the ring so if you tear you glove the suit wont flood.
Someone already mentioned the zipper placement. I have an across the back shoulder zipper and depend on my buddy to zip or un-zip the suit. My next suit will have a zipper that I can work to eliminate the dependency.
My suit is a Mobbys tri-lam shell suit. The tri-lam just refers to the construction of the air tight enclosure. The shell refers to the outer covering that protects the inner suit. It is sort of like wearing coveralls over your normal clothes. This is the main reason I am planning on getting a new suit. The shell has shrunk (not around my waist! but crotch to neck). Until I get the shell wet I am kind of uncomfortable. That being said, fit is extremely important. If you do consider a shell, when you are trying the suit on at the shop take into account that a wet shell will get bigger . Both my wifes and my suit end up dragging at the ankles after a dive. It was a perfect fit when we bought them.
Check for added reinforcement at the knees, seat, and any other point that might get rubbed. Determine if you want pockets on the suit too.
Fins may have to be upgraded too. My warm water fins had pockets that were too small for my drysuit boots.
The last thing I can think of is boots. Drysuits come with the boots attached or designed with booties that require boots to wear over them. The Mobbys boots are attached and work fine but the soles are kind of thin and walking to and from the water I can feel just about every thing I step on.
By the way, after you get your suit and go diving, just watching some wet suit divers trying to warm up after a dive when all you had to do was towel off your head will confirm you made the right decision to go dry.