As jrista said, almost any lens can be fixed, but you have to compare the cost of repair against the value of the lens.
Nikon will often charge over $200 for repairs, but you can probably get a lens cleaned in most parts of the world for $50-100. You have to weigh that cost vs the cost of the lens. Repair or cleaning might cost most of the value of a kit lens, but be worthwhile for a $2000 professional lens.
Mold can be prevented by regular use of your equipment. UV light will kill the mold, so having your lens outdoors periodically should prevent it (but won't remove it once it's there). A small amount of mold won't generally ruin the IQ of a lens. Unless it's really bad it usually only causes loss of contrast or some flare, but most times it's not noticeable. A dry box needs to be weighed against the value of the equipment it's protecting, and it depends on the humidity of your location whether it's even necessary.
The other main preventative measure is having a filter of some sort on the lens. This can help protect the front element of the lens from scratches, or possibly protect the front element or outer rim/filter threads if the lens is dropped. Whether this actually helps depends on how you drop it I guess, but it's debatable whether it's worthwhile. The problem is a cheap filter can degrade the image quality (do you want to put a cheap $20 filter on a $1500 lens?, but an expensive filter may not be worth it (do you want to put an $80 filter on a $150 kit lens in hopes it might protect it?). Some debate about this topic can be seen here.
Other things that can go wrong: the most common would be autofocus mechanisms that stop working, aperture blades that get stuck, zooms that don't zoom smoothly. These can result from droppage, moisture, or old age.