gis usability issues

Let's start with consistency. Consistency is about making similar things look and behave similar!
By being consistent you provide users with a good chance to learn new contexts (and to detect new contexts, of course), to concentrate on relevant tasks, to feel safe.
There are several different forms of consistency and we might as well start with aesthetical consistency, which is about providing a look & feel that is familiar or will at least be recognised again easily and thus creates emotional expecations. Consider ESRI websites, for example.
Websites should look alike and be found the same way accross the world. For example, if you know that the ESRI website in sweden is found using the URL http://www.esri-sweden.com, then you should expect that spain is found under http://www.esri-spain.com. Well, that sadly, is not the case.
Likewise, take a look at the following two snapshots. Don't so much look alike, do they?

 

User should be able to use existing knowledge to learn how to use something, i.e. similar functionalities should work the same way. For example, you know that a traffic light shows yellow before it turns red. Likewise you know that a light shows that your computer is turned on.

What about adding layers / shapes / relations to your working area in GIS? Works different in all GIS. How about projecting a layer? How many coordinates and specifics can you / do you have to enter and in what sequence?

External consistency is naturally very hard to accomplish. Vendors / developers would have to co-operate very much and in some cases even make public their algorithms to achieve this. Not a very likely thought.
BUT, it can be done!! This is where the Open Geospatial consortium® comes into play. Their ultimate goal is to design interoperable GIS where a user might use one part from one GIS and buy additionally needed parts from other GIS.

 

Well, if it is unlikely for the next years to expect sound external consistency, shouldn't we expect that at least internally the consistency is right?!

In the shortcuts-box above you can see that this is not the case. Especially ArcView breaks out as they offer the same shortcut several times, so you have to learn the meaning not only once but several times, also always keeping in mind in which document object you are working, i.e. view, table, dialog designer, script, ...

There is so much more to say and discuss, which I will add here, so make sure you come by again. Or, meet me at my blog.

 

An issue that walks the same path is: 'Control of a system should correspond to the abilities of the user'

This means that the interface should adapt to the needs of the user. A novice GIS user will naturally require guidance and well-designed introductions that help him / her explore the yet unknown user interface step by step.
An expert GIS user will require that no impeding questions be asked like 'are you certain you want to that?'. Instead, the expert needs shortcuts and even commandline interfaces at hand to be able to act as quick as possible.
The image below shows that. Help the novice user not to fall whereas help the expert user to run at his / her own speed.

The question, of course, is how many user groups to take into account. There might be novice and expert users, but there are bound to be many more levels of experience. Well, I would say, keep the interface as simple as possible and do not take more than these two groups into account.