RomanceLove is always in the air in a household with a Romance-oriented Sim. These libidinous individuals want nothing more than to have multiple romantic partners and, if possible, to make Woohoo with multiple partners, as well. Living in a blissfully STD-free world as they do, the only thing they really have to worry about is jealousy. Their insatiable desires can lead to friction within a household, though, especially if they're partnered up with a Family-oriented Sim.
So far as specific Fears go, though, Romantic Sims will have a congenital dislike for the heavier side of relationships; they don't want to have children, get married, or even to Go Steady while they're teenagers. Anything that ties them down to a single person will be repulsive to them. Again, this makes them terrible mates for Family Sims, since one partner's Wants will often be the other's partners Fears.
FortuneA Sim that desires nothing more than Fortune will, of necessity, have to focus on their career first and family second, if at all. Most of their Wants will come in the form of gaining promotions, making money, and buying expensive items for the house; in short, they want to live the high life. Their most powerful Wants involve the upper echelons of wealth. Whether they marry into money, or simply earn an astounding amount of it through their own sweat and toil, nothing less than a bank account full of Simoleans will satisfy these Sims.
Luckily, the Fortune Aspiration is easily compatible with those of other Sims, so they are good choices for marriage partners and breadwinners. Most of the Fears of the Fortune Aspiration are commonsensical things that won't translate into Wants for any other type of Aspiration; things like having items repossessed or stolen or being fired from work will reduce your Aspiration meter, but then, you should be avoiding these events in the case of all of your Sims, not just the ones with dollar signs in their eyes.
PopularitySome Sims are concerned more with being the center of attention than anything else; these characters usually possess the Popularity Aspiration. These Sims desire two things above all others: to have a lot of friends, and to throw fantastic parties. Thus, most of their wants revolve around these two activities. If you plan out your house for efficient socializing and so that parties can go off without a hitch, you should be able to satisfy many of these wants without a problem.
Popularity comes somewhere in between the Romance and Family Aspirations when it comes to how much you're going to be socializing and with whom. You will need to keep in touch with friends from outside the family if you wish to achieve some of the higher-end Wants, which require a lot of friends, but you can also benefit from having strong relationships with other members of your family. Note also that most of your wants will require mere friendships with other Sims; romantic entanglements are left for the Romance and Family Sims, while a Popular Sim can function just as well as a celibate as they can if they're wantonly promiscuous. Since you can gain many of your Wants by just having Friends, this is a good Aspiration to pair up against a Romantic Sim. If you're sharing a house, the Romantic Sim can attempt to woo all of your guests, while the Popularity Sim can socialize and make friends with all of them, without risking a terrible amount of jealousy between the two.
KnowledgeThe writer in his garret, the scholar in the ivory tower, the Sim trying really, really hard to learn how to clean up messes; these are all examples of individuals with the Knowledge aspiration. In Sims terms, though, the intellectuals require nothing so much as they do constant skill increases. They do want some other things in life, including obscure experiences, such as seeing a ghost or having a near-death experience, but in general, a Knowledge Sim will see his routine involve a whole lot of skill training.
Since skills require quite a bit of time to increase, though, a Sim with the Knowledge Aspiration will require high Need scores so that they don't autonomously break off their studies and choose to do other things. If you want to have your would-be scholars concentrate on their books, then, you'll need to be sure that they have enough time to work on their Needs, as well. As such, it's best if they don't have a job; it's possible to juggle constant skill-ups with obtaining Needs and holding down a job, but it's difficult at best, especially when you have to go above and beyond the skill acquisitions required for your career. If you do decide to enter the workplace, though, you'll probably have an easy time getting promotions if you keep your job performance up.
Since Knowledge Sims will spend so much time on their skill improvements, they won't have many Wants related to romance or social interactions. Thus, they perform well as single Sims, without the hassles of family life interfering with their quest for truth.