I think that's pretty well documented if you do a search or look around here. There are few books on that also.
On personal rant, The problem is not the user interace it is your convoluted business process that is making your system unfriendly and your user's unwillingless to enter a minimal amount of data causing the problem.
Sorry but a business process can't be a simple as a facebook or twitter post, because those only capture one data element. On the otherhand it is extremely ridiculous to expect people to remember how to enter 30+ fields as part of a process they only do occassionally. I don't care what UI you are using you won't solve your problems.
However the problems are that companies want to not invest in training $$$ on their employees for complex processes and employees feel entitled that work stuff needs be as easy as using social media. IMHO the SAP GUI is very efficient in doing repeated work, if you are properly trained and not too hard if you spend some effort. I'm sorry but tell your younger employees who complain about UI that if they don't like it we can switch back to 3270 terminal emulation(cloud based of course 
.
That being said if UI is really an issue, then go for the SAP Customer on Demand solution(pretty darn impressive I must admit) and it has a better UI than salesforce.com .
Take care,
Stephen