Fantastic game, best of the series.
The graphics looks exelent, they have personality to them now, and even simple actions that were meaningless before actually have a purpose now. For example, using a mirror to "Gussy up" didn't really have much purpose in Sims 2, but now you get a mood bonus as a result. There are far more interactions with other sims, and you can visit them anywhere. Another nice change is you no longer have instant crushes with every sim you try to make friends with. Initiating a romantic relationship requires specific interactions in order for the relationship to go along that path.
Jobs are far different with adjusting how you want your sim to work (or school for children/teens) and the requirements for promotions are different. Some careers even branch, for example Law Enfocement eventually splits between Forensics and Special Agent. No longer must you have specific skills and number of friends. Different aspects related to your job have positive or negative effects on job performance. Job skills are a range to get, having too little is a negative effect while having the highest or above has a large positive effect. Depending on the level you can have from 1 to 5 different things effecting your job performance.
Aranging furnishings is much nicer, with chairs linking to tables so you don't end up with a click fest when trying to rearange things. Everything can also be recolored in game with just a few clicks. Paterns can even be put onto whatever you want, you're not stuck with whatever happens to be available or downloaded any longer.
There are a few bugs though, sometimes sims have problems finding their ways around and have to be guided. Some other functions of the game also need work, but they are minor and don't cause a major problem with gameplay.