This is the key item that allows a diver's eyes to see properly under water, so it pays to invest in a good mask. Getting a cheap $25 snorkeling mask will leak more, fog more and is more prone to scratches and skirt tears. A mask like that will detract from, or even ruin, plenty of diving trips, so it pays to pay a little extra to get a good mask. The other basic consideration about scuba masks is how they fit, which will also directly impact dive quality. A mask that fits well won't leak as much, won't fog as much, won't give the wearer a case of "mask face" and will last longer.
It's important to understand that while divers do wear snorkeling fins or swim fins on dives, especially if they are renting their gear from a dive shop, proper scuba fins are bigger and often have added features that allow them greater propulsive power. Getting the most propulsion out of the least amount of movement is critical to divers because that helps reduce air consumption. In the short term, it is OK to cut costs and get a cheap, basic pair of fins. In the long term, however, those should eventually be replaced.
The two most important considerations when buying a wetsuit are how it fits and if it is suited to the waters that will be dived most often. A loose wetsuit might be easier to get on and off, but it also does not do its job of creating a trapped, insulating layer of water and keeping the cold out as well. Past that, it's all about how thick the wetsuit needs to be to keep a diver warm. Divers in Florida can dive with 2 mm shorties, but in North Carolina they need a 3 mm or 5 mm full wetsuit, and in New England, a 7 mm, semi-dry, r dry suit is required.
In addition to its function as buoyancy adjusters and flotation devices on the surface, the BCD also is the anchor point for most of a diver's gear. It is important to get a good fit and to be comfortable with where all the hooks and straps are. Also, pay attention to lift capacity and the empty weight of the BCD. Lift capacity determines how much extra buoyancy the BCD give you when it is full, and as some divers take more gear than others, they also need more lift. The empty weight is an important issue for any diver who plans to travel a lot to reach dive sites, especially if that entails frequent flying.
The regulator is the most critical piece of gear for a scuba diver since it houses the valves that allow a diver to breath compressed air at different undersea pressures. It is no exaggeration to say that a diver's life depends on the reliability of his regulator. While that does not mean a scuba diver who is assembling his/her own kit needs to go out and buy the most advanced, most expensive regulator they can find (not unless he is diving shipwrecks off Norway in 200 feet of water anyway), it is possible to get a good mix of quality and low price at the same time. Quality should always be the first consideration, and the price tag should be a distant second.