I consider a UI good if it's functional, intuitive, user friendly, and looks good. Here are some examples of good or bad design:

• Redirecting the user to the question list page after a user adds an answer is bad.
• Not having the logout link on each page is bad.
• Hiding the vote function if the answer is given by the current user is good.
• Hiding the + (or -) vote function if the user already gave a + (or -) vote on the answer is good.
• Hiding the input forms before a user logs in is good.
• Plain UI is OK (for homework), but ugly/messy ones are bad.

abajpai wrote:

Is it part of the required functionality that a user can't change his/her vote once they cast it? Because there was a discussion about this in the forum earlier, about what happens when a user tries to change his/her vote, I thought that it was allowed for the user to change their vote.

Yes, a user is allowed to change their vote, but remember that a user can only give one vote (+ or -) to an answer. If the user already gave a + vote to a answer, the + vote operation can be hidden to indicate to the user that they cannot vote + again. The - vote function will still be visible to let the user change their vote.

Hey professor, i have another question. If a user gives a positive vote, then a good UI would show only the -ve vote option and not the +ve one. Now if the user clicks on the -ve vote button, should it give him the options of both +ve and -ve or just directly show only the -ve one again?

So which of those 2 behaviors should be used to get the extra credit? 

or, you could simply display the one that is useable as a link, and the other as text, that way, you keep both elements, but you can still control which the user is able to select.