Ultimately it comes down to the sound, but all the other factors come into play as well - including a few not on the list, build quality, customer service, and price.
While asthetics aren't everything, they do play a part in the emotional connection to the instrument, and I am always amazed by the lack of asthetics in most bass designs. I am extremely picky about what I like and don't, although I have been known to let some designs grow on me that I initially resisted.
Feel is very important as well, an uncomfortable instrument will not be played much, and therefore has limited value. Sometimes one has to endure some discomfort for a specialized sound, but it isn't something that you are eager to deal with.
Sound is critical. I've played a number of guitars/basses over the years that measured up in all but the most important area. While some can be "fixed" by changing out electronics, most of the time it isn't worth the hassle/expense.
Build quality is extremely important, especially once we get into premium instruments. I don't want the guitar to be falling apart or have to be dealing with twisting necks or other mechanical/electrical problems. I'm not one for adornments, I prefer a well crafted piece to one with excess flash, but a nice piece of wood never hurts.
Customer service is extremely important. With any manufactured item there is potential for problems. How the company deals with them is critical to my desire to continue buying their offerings. Poor customer service is the fastest way to lose support for your products. Fortunately that doesn't seem to be an issue around here.

Finally, price. There has to be a realistic balance between what is paid and what you get. I don't mind paying for quality, but if I can afford two for the price of one it's always better...