The only reason we call them challenges is that everytime you focus on optimizing one of them it affects the performance of one or more of the others. Here are a few of the big things to think about from an engineering perspective.
If the structure is being built from scratch it not such a big deal. The trick is that the top of the opening has to have an engineered "header" that can support the roof and other floors that are above it. The header is a heavy wooden beam or steel member supported on the ends that supports the weight in a downward direction and windload in a horizontal direction.
When a blank wall or wall with a few "punched" windows or doors is being planned to open up to a window wall, there is usually no such header in place. This means that the hole has to be carefully cut out while the wall above it temporarily supported while the new header is put in place. This is a big deal and brings many "open the wall up" projects to a screeching halt once the challenge is discovered.
With most windows walls the goal is to maximize the glass area, but the non-glass material making up the windows, doors and, mullions is also important. These are often called the profiles. The most common profile materials are:
Wood comes in many species from pine to mahogany with each serving a different durability and aesthetic objective. Wood ofter requires continued maintenance with painting and it can swell and shrink over time. That being said, wood remains one of the most common materials for the profile because it is a natural insulator and relatively less expensive.
Aluminum is the most common metal profile material because it can be easily extruded into intricate shapes. It is most common in commercial buildings and for making sliding and folding doors. The addition of thermal breaks in the 1970's overcame the major objection which was poor thermal performance. It has better stability than wood and it stronger than any profile material other than steel
Vinyl is the plastic windows that dominates the replacement window market in the U.S. It has very good thermal values. It's relatively weak structural performance can be improved with multiple chambers within the shape and though the addition internally with steel reinforcements. Another advantage is that the corners are thermally welded which is stronger than wood. The corners are also more stable than mechanically joined aluminum corners and seal better. Color choices are usually limited with vinyl.
Steel window and door profiles date back to the 1800's when hot rolling of steel was perfected for the creation of railroad tracks. Steel is naturally stronger than any of the other profile materials. This makes steel very desirable for ultra-modern structures that feature glass area at all cost. With steel being a better insulator than aluminum, thermal breaks in steel sections has only become popular in the last several years. Steel profiles are painted after welding so color selection is limitless. Rust is the enemy of steel, but modern finishing techniques have gotten this issue under control
We've talked about thermal performance for the various profile systems above, but the profile is usually only 10-25% of the window wall opening. The glass is the larger element affecting the overall thermal value of the wall.
Insulating value is rated in "R" values which are the reciprocal of thermal conductivity. Single glass is about R=1 while nominal insulated glass with two panes of clear, uncoated glass having R=2. Triple paned insulated glass improves the thermal performance even more.
Low emissivity (Low-E) coatings can be added to one or more surfaces of either the inside or outside pane of insulated glass to improve thermal performance. The space between the panes is not a vacuum as some people think but just simple dehydrated air. The air can be replaced with an inert gas like argon to improve the performance even more.
Maximized insulated glass (IG) with all of the tricks utilized can bring the overall IG thermal performance down to R=5, but the cost starts to really go up and clarity of the glass goes down as the thermal performance improves in most cases.
Especially when doors are involved, there is also the battle over water infiltration and trip hazard as it comes to sill design.
Door sills that seal best stopping air and water usually require a compression sealing gasket that allows the moving sash to sandwich a flexible rubber material between itself and the fixed frame to stop both. Compression seals are only on swinging or projected window and door elements. Sliding elements need sliding gaskets which by nature don't seal as easily.
Compression seals and most sliding seals require a step over. If the door opening is being used to join the room to the all outdoors as when large sliding or folding doors are used, the most desirable solution is to have "zero" saddle conditions where the floor and deck are both on the same plane. This is a major challenge especially for stopping water infiltration. Troughs below the floor surface can carry water that enters to safely pass to the outside, but not foolproof.
Screens are the bane of existence for window and door designers. Insect screens obstruct the entry of light and reduce the clarity of vision. The can be fixed, sliding or swinging in function and some even roll up into canisters. Except for the roll-up or pleated varieties they never go away. For full opening movable glass walls a decision needs to be made whether to keep the bugs out or just go natural and open up everything to free movement between the inside and outside.