The problem with wide columns is that even though SQL Server supports now larger row size the maximum capacity specification is still 8,060 bytes per row. If the row becomes wider - then row overflow occurs and data saved differently. This affects query performance (I/O performance degradation). More details are here http://msdn.microsoft.com/en-us/library/ms186981.aspx. This works differently with varchar(max), nvarchar(max), varbinary(max), text, image, or xml data types, but I just wanted to make point that columns width should be reasonable.

Another example of incorrect data type usage is using NVARCHAR or FLOAT data types when it's not required. Just a couple of days ago I have been testing large database migration and by changing NVARCHAR, DATETIME to VARCHAR and DATE/SMALLDATETIME (together with column width reduction) one of the tables reduced in size by 11 GB! There were columns like Year nvarchar (255), PostalCode nvarchar (255), Phone nvarchar (8000)...