First, have your BCD and reg serviced annually (or as recommended by the manufacturer). There are some parts that do wear out, and will need to be replaced periodically. Reg hoses should be replaced periodically too. Properly serviced BCs and regs should be good for several hundred dives. Your diving frequency will determine how long that is.
Rinse all gear in fresh water, and allow the gear to air dry out of direct sunlight. UV rays in sunlight can degrade some gear components. (Some camera gear should be towel dried according to manufacturer recommendations.)
Masks will need to be replaced as the skirt will become oxidized over time. Silicone mask skirts last longer (and usually cost more) than rubber skirts. My silicone skirted mask is going strong after 7 years and hundreds of dives. A snorkle is probably good for as long as you don't lose it.
Certain fin designs may develop cracks over time, particularly if abused. Drop a tank on a stiff blade fin often enough, and you will ruin it.
Wetsuits may need a washing periodically to remove certain odors. Store wetsuits on a hanger designed for this use, and avoid folding. Repeated use will degrade the insulating capacity of the wetsuit, as the small bubbles in the neoprene will rupture over time. I try to replace my wetsuit after about 200 dives. Your milage may vary.
Tanks should be visually inspected annually, or any time when you suck it dry and water may have entered. Hydrostatic testing is required every 5 years (or more often if required by law). Properly maintained tanks (especially steel ones) may have service lives of 30+ years.
Computers, SPG's, compasses, and other gauges may fail over time. I have heard anecdotal stories that oil filled gauges fail faster if repeatedly exposed to air pressure changes experienced in flying. Don't know if that is true, but I did have to replace a compass when the oil leaked and the needle became stuck. I had to replace my really old computer when the manufacturer stopped replacing the batteries. Most computers now have batteries that the user can replace, so that may no longer be an issue.