+3 on the service and repair first suggestion. If you aren't comfortable or don't have the resources to do it yourself, a good LBS can do a basic lube n tune for about $50 plus parts (if any). They can also point out potential problem areas and help you with proper fit. This is also the time to replace parts that are worn out or just don't work adequately.
I'd put safety items #2a like helmet, safety glasses ( polycarbonate tinted or clear ), and gloves ( comfort item but also prevents handburger if you crash ). Tires can be considered a safety item if your current ones are inadequate. If you're riding in twilight or after dark, high visibility clothing and front and rear lights are also high priority.
As mentioned, proper fit is right up there at like #2b as fit is both a comfort and safety concern, as well as performance issue. If the bike is the proper frame size, you can usually make adequate adjustments for safety and basic fit. Then ride it for a while as you determine if you need to make changes to things like the seat, seatpost, stem, handlebar, or crank length.
At #3 I'd place utilitarian upgrades that make the bike more suitable for its intended purpose. Tires can also fall into this category as do things like fenders and a rack for a commuter, bottle cages and tool bag for road riding or touring, bar ends or special grips for MTB or climbing, etc.
#4 performance upgrades. When I wear something out I try to replace it with something better. Each individual will have to determine how far up the component hierarchy makes sense for their budget and needs.
#5 Bling. Those unnecessary items you get simply because they are cool. Bling can be blatant or a matter of perspective. For example, $2,000 wheels can be bling for the average rider looking for oohs and ahhs at the local club rides, but can mean the difference between first and second place to a professional racer.