A visual Ultimaker troubleshooting : Overhangs / Very visible lines on the bottom layer

Overhangs 

The reason overhangs come out uglier than a straight wall is simply because new layers are not properly supported by the preceding layer. Rather than fully resting and being anchored in place by the previous layer the new layers are partially printed into mid air and tend to sag down slightly or curl up. Sometimes these issues accumulate making each layer worse than the last. Curling around corners when using thin layers seems to be especially problematic. Dealing with overhangs is tricky, there are many variables that will affect how well or badly they will be printed. Temperature, print speed, amount of overhang, layer height, material, and cooling all play a part in how an overhang will print.

Like so many other things cooling plays one of the biggest roles in how well an overhang will print. Ensure that your cooling fans are going 100% when the overhang is being printed. If the object you're trying to print is small there's a chance that, due to the way the nozzle is positioned, the right fan never gets a chance to properly cool the print. A prime example of this is the right ear of the Ultimaker robot. A way around this is to print more than one object at the same time. By doing this the print head will move between the two objects and allow the layer of one object cool down while the same layer is being printed on the other copy. This also helps greatly when the layer currently being printed is very small. When printing very small details such as the antennas on the Ultimaker robot the print head will stay over the same spot for quite some time and transfer a lot of heat into the print which will deform the layer quite badly.

Another variable is layer height. Depending on your print, sometimes a thicker layer height will be helpful in improving the quality and sometimes a thinner layer is helpful. Thinner layers seem to create a more pronounced upward curling of the edges and especially around sharp corners. You'll simply have to experiment and see what works best in your situation.

Print speed will also affect your print quality. Slowing down will usually always result in an improvement.

Try to reduce the print temperature as much as possible without causing under extrusion. The slower you print the lower your print temperature can be. In addition to reducing the print temperature it can be worth lowering the print bed temperature, or even turning it off completely. This is especially important if the overhang is close to the bed.

If at all possible try to orient your object to minimize the amount of overhangs. Look at your model and imagine how the print head will have to move and then try to figure out how to rotate or angle it to make this easier. If you are the creator of the model do your best to avoid overhangs or reduce their severity. If it's possible to keep an overhang to 45 degrees instead of 30 that's a good thing as it will be much easier for the printer to handle. You might also consider changing your design to allow for a bridge rather than an overhang.

The top object has an overhang that can be quite hard to get a nice clean surface on. The bottom object has replaced the overhang with a straight "roof" that can instead be bridged which can produce a cleaner result in some cases. Bridging is when the print head will print straight across a gap between two islands into mid air. This actually works better than you might think, especially if the jump is short.

There is a limit to how much of an overhang you can print while still preserving the quality you want and this is simply a limitation of the type of printer the Ultimaker is. Where this line is drawn depends on your own expectations, what plastic you are using, what the geometry of the overhang is, how well it can be cooled and many other factors.

Very visible lines on the bottom layer 

If the bottom layer of your print is showing very obvious print lines it's likely that your bed is simply levelled a little bit too far away from the nozzle. The closer to the nozzle the bed is on the first layer the harder the plastic will be squished into the bed and the lines will then blend together better. However, you can't go too closely as that will prevent the plastic from escaping from the nozzle properly. Pressure will build up and eventually the plastic will squirt out and create an ugly blob, or, it could cause the feeder to grind your filament which is something you don't want.

By default cura will print a 0.3mm thick first layer. You may want to try reducing this to 0.2mm or even 0.1mm to make the bottom layer even more of a mirror like surface. However, when you reduce the initial layer height you must be very precise in your bed levelling. A 0.3mm layer height is quite forgiving, a 0.1mm layer height is anything but.