Thank you for the feedback! Here's a few answers I hope might help you out.
"1. If you make too tight a turn (which of course you tend to do to save a much money as possible) the system will create the tunnel but there will be a break in the track which you will almost certainly not notice until it is too late. When you try to fill in the missing track you get error msg "Turn is too tight". IMO it should not let you make a tunnel if you can't have the track as well."
I'll see if this can be done. Generally, when we decided to have a very flexible road and track system, it also meant that there are going to be some instances where it's hard to make the results coherent. I'll be passing this on to the programmer and see what can be done.
"2. It is very, very difficult to get the end of the loop to join successfully back onto the main track. Sometimes it looks like it has but actually hasn't, sometimes it just won't join up where you want it to, or even near where you want it to, for no apparent reason. It usually gives a message "There is no room to build here" but there is no apparent obstruction or reason for this. This doesn't feel right."
Joining the tracks can be difficult. You can also use another depot to turn the metro train, or build looped lines to avoid building the small loop at the end. Large loops are much easier to make in my opinion.
"3. Both 1 and 2 are exacerbated by the schematic overlay of the road system above in the underground view which often obstructs your view of your tunnel and track. It would help to make the "U" command a triple toggle between standard view, underground view with road schematic, and underground by itself."
The underground view is a known problem and we are looking for a solution to it! It should make to the next patch, unless fixing proves to be very difficult.
"1. Buses not leaving the depot. I have a line where only 2 of 7 buses will operate, the rest won't drive the route at their assigned times (even if I change the timetable), and my stops are full of angry people! The buses in the depot are all at 100% and should be ready to go... I even bought a few more to see if that would work, but no. I consider this enough of a showstopper that I´ve temporarily stopped playing the campaign."
Like lightrail said, this is most likely because the line is not closed. The depot must be the last click when adding stops so the buses find their way back.
"2. When I load the first campaign mission, it says I´m 20% done with the first objective. But then as the game progresses a few minutes it drops to 6%. I don´t understand why?? I think I've covered at least 25% of the city with my lines..."
Have you looked at the coverage graph in the data panel? We wanted to make the coverage quite realistic, so it's not actually how much of the city is geographically covered by lines, but rather how many of the trips inside the city are made with your company's vehicles. So you need to have lines that meet the citizens needs. The percentage can go down because the game system calculates the routes that are actually used, but it will likely rise up again at some time of day when the routes are the most used.
"3. I know there's been a mod posted for this by a user to allow for more traffic, but I would really like to understand why most roads are so empty on the standard game setting. It seems to me like you went through all this trouble to build huge varieties of roads, from one-way to three-lane and expressways and even bus lanes, and then hardly anyone is using them. I think looking at the traffic overlay, I've seen a small traffic jam maybe once. The city just seems empty, and not at all "in motion"! Please note that I do understand that adding more traffic would take up more computer power and some computers may not be able to handle that. So maybe this should be a setting in the Options panel? Or maybe you could make it so I can't follow ALL cars and people around town, but just certain ones, and simulate the rest of the traffic? Either way, having almost-empty cities takes some of the fun out of the game for me personally - the cities seem like they've been overrun with zombies five years ago, and that just doesn't look realistic. Plus, I miss some of the fun I had in CiM1 with trying to work around traffic jams, something you actually tried to help with in CiM2 by adding waypoints. (But waypoints seem completely unnecessary for me right now, because there are no traffic jams to dodge.) I know that CiM1 had issues with too much traffic, but it seems for CiM2 the opposite is true. Please, please fix??

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When you start a game, the city is created and it takes a while for the citizens to start finding their way around. When you play the game, the cities get more lively. The large map size means there are bottlenecks in cities, that could eventually clog the whole road system, so there is a moderate amount of cars in each city to avoid this. With the map editor you can choose how many of the citizens own cars, so you can make much busier cities. You can also check the custom maps section on the forums if someone is sharing a busy map
"4. Although definitely not a showstopper, I too am interested to see why things like train lines and airports have been removed from the game. For me, in CiM1, they were very reliable starting points in that I knew that they would always attract a decent number of passengers to get my lines started. Additionally, some kind of connection to the "outer world", although abstract, would just add to the realism of the game."
These are all good ideas and we will keep them in store! Right now we are concentrating on making all the fixes as fast as we can, but we would like to add more to the game later on.