I agree that modern GPS systems are still a bit antiquated when it comes to the voice instructions. The voice usually sounds robotic like a text to speech program from the 1990s. There are many little things about them that make it annoying to deal with all the time.
Many of the names for cities, routes, etc. are often mispronounced. There are many bugs such as one of my GPS' says 9 grams when I am supposed to turn onto 9G. A new voice, combined with improved pronunciation would go a long way. The voice can also be abrupt as well, so having the voice start out more softly, or with another non-vocal cue would help.
Perhaps a very calm tone could alert them that there is a turn ahead so they know to look at the screen. The on-screen instructions could be improved quite a bit, and be updated in real time. The use of more colors, better animation, etc. would be a big improvement. For example, the turn arrow could gently blink as you are approaching your turn. If there are multiple turns close to one another, it should also display at least one turn ahead so the driver has enough time to react.
Another issue is that they give false information. It may say to turn, but in reality, you are staying on the same road, and there is an exit that it gets confused with. They also get very confused with tight turns and cul-de-sacs.
The routing algorithms also have issues. They tend to send you on a wild goose chase on tons of back roads, and sometimes they are unpaved just to save a tenth of a mile or so compared to just staying on the main road. They should be setup in a way that will always try to route main roads, and only go on back roads when is completely necessary. It should also keep track of the number of turns, and reject routes that have excessive turns. The GPS software which is used by some major courier companies knows to avoid certain patterns such as left hand turns because it takes longer to turn left than right. It can also be tuned to avoid traffic lights, school zones, business districts, etc. and stay on the truck route.
It is also difficult to input information into them. The driver should be able to control it all by voice commands. Lets say that they want to go on break and they are looking for a fast food place. They should be able to quickly, and easily add that destination to the route as a waypoint without disrupting the rest of the route. Lets also say that they are 5 miles away from the closest fast food place, and there are still stops along the way. Normal GPS systems will just add the new destination as the next stop. If it does that, that means you will have to backtrack. The algorithm should figure out when the best place to insert that destination. It should also handle multiple stops in the middle, such as for fuel, and food at different locations.
All of these features would be great for consumer models as well. It seems like the GPS market has stagnated quite a bit since many newer cars have them built in. Many others simply rely on their phones.