How hard can it be not to install wires? Well, a lot harder than you might think. If you haven't been working with wireless very long or perhaps if you have been working with wireless for a while, but haven't really dived and how it works, you might be making invalid assumptions. Wi-Fi is it pretty robust protocol, but depending on your environment, the ability of its robustness to hide your design mistakes varies.
The biggest and most common design mistake is not actually designing the network. Radio frequencies (RF) don't always move the way you think they should. While RF does move very much like light, unlike light it has the ability to pass through solid objects to varying degrees. RF often travels much farther than you think. For example, just because all your access points are contained within a building does not mean that all of your RF is contained in the building, too. Most buildings will bleed Wi-Fi with no problem. Whether or not that is an actual issue depends on how you've configured your WLAN and on your policies/desires. For example, let's say you have a guest wireless network that is intended for your clients to use. Perhaps there are other retail or professional offices next door to your location. If your wireless has not been designed with your intended coverage area in mind, you may well be providing free Wi-Fi to the users in those offices and their clients.
An example of this is a location I managed that was surrounded by nearby apartment complexes. Network monitoring told me that even after that location closed, there was quite a bit of use. Since all of the access points were running at full power and the site had not been properly designed, I was unintentionally providing free Internet access for the residents of these apartment complexes. In fact, I later found out that several of the residents had even built directional antennas out of Pringles cans to improve their signal. I found this out after the site's design was improved and they could no longer pick up a usable signal because they came to investigate what had happened to their free Internet! The biggest change there was bringing the power levels on the APs down to a fraction of what they had been, since that was all that was needed for the small site. Design doesn't always have to complex
New construction in commercial buildings will usually have a number of locations labeled "WAP" on the blueprint that have been placed by the architect. Note: the architect is not an RF engineer. The architect has a rule of thumb, possibly even a standard that he is following, that tells him there should be an AP for every x square feet. This rule of thumb does not take into account the construction of the building. A modern building that is constructed with steel beams, aluminum studs, lots of glass and an open ceiling plan with exposed HVAC is going to require a very different Wi-Fi design than a more residential style stick-built building. The denser the material, the more RF it will block. Usually, you end up with APs in all the wrong locations and you may even end up with far more access points than are actually required for the site.
In these situations, everything is installed and turned on and the network does not work to the satisfaction of users. Some areas will be dead spots and others will have too many APs. Money ends up being wasted because the wireless work will have to be redone. In those situations, hopefully the APs can be moved to better locations, but sometimes you may be stuck with making it work as best you can in the existing locations. In the most wasteful scenarios you may even need to turn half of the access points off completely because having too many access points is causing them to interfere with each other.
Okay, you get it. Wireless networks should be designed. How are you going to do that? Ideally, you own a wireless design tool such as Ekahau Site Survey or AirMagnet Survey Professional. However, neither of these are inexpensive tools if WLAN design is not a task that you are doing regularly. I recommend you hire a consultant who knows how to use these tools to create proper designs. These are usually a "predictive site survey," a mathematical model of the building that makes a guess as to how RF will propagate. You should also check with your VAR, because they may be able to provide design services as part of a wireless equipment purchase or at reasonable cost because you are an existing customer.
If you really want to dive into WLAN design, I suggest checking into the CWNA and CWDP certifications from CWNP. These certifications will both go a long way towards helping you learn proper WLAN design and understanding how Wi-Fi really works. That will help you recognize and solve problems thus improving the networks you are responsible for. And really, isn't that our goal?