WebEditCommand?!

The WebEditCommand class once resided in the Sitecore.Client.dll, as of Sitecore 8.1 it has moved to Sitecore.ExperienceEditor.dll.

Issues:

The type or namespace name 'WebEditCommand' could not be found (are you missing a using directive or an assembly reference?)
'xxx.Execute(CommandContext)': no suitable method found to override	

Solution:

Install the Sitecore.ExperienceEditor.NoReferences NuGet package

No more Datepicker

Sitecore 8 introduced the ability to pick a time as well as a date.

Issues:

The type or namespace name 'SelectDateOptions' could not be found (are you missing a using directive or an assembly reference?)	

Solution:

Rework code to use DateTimeSelector.

Applications namespace does not exist

The Sitecore.Applications was removed, it looks as if the only class that existed there was the SelectDateOptions.

Issues:

The type or namespace name 'Applications' does not exist in the namespace 'Sitecore' (are you missing an assembly reference?)

Solution:

This can be removed as any code that has this namespace import should be reworked to use DateTimeSelector.

LoginPage namespace does not exist

This issue came about as the default Sitecore login page had been extended with bespoke business requirements.

Issues:

The type or namespace name 'LoginPage' does not exist in the namespace 'Sitecore.sitecore.login' (are you missing an assembly reference?)

Solution:

The Sitecore login page now inherits from Sitecore.sitecore.login.Default, so you can either update the Inherits attribute of the aspx page of your customisation to this new type and address any other compilation issues, but I would strongly recommend not to take this path and create a new page instead. Avoid modification of default Sitecore code as it can cause headaches when upgrading and debugging.