Accessibility: Everything should be in easy reach of your hands. The dumbest place you can put your big macho dive knife is on your ankle. Those big snap-clips are lovely and easy to put on, but can be awkward to unclip - that's not a good thing. Stab jacket pockets tend to be inaccessible, thigh-mounted pockets are much handier.
Danglies: Avoid letting bits of kit hang off you. Every single item you carry should be secured in an exact location. A dangling item can get wedged, come off without you noticing it, and be hard to find. The single most important item to secure firmly is your backup demand valve/octopus. If you can't lay your hand on it in under a second with 100% certainty, then don't get in the water.
Read around: You stand a much better chance of getting decent kit if you know what you're looking for. Never rely solely on your dive shop for advice - even if you get an honest dive shop, their own personal preference can cause bad advice. For instance, my local dive shop always recommends cuff dumps on dry suits. They truly believe them to be the best way to dive - they use them themselves. But they're in the minority. Get as many opinions about everything possible. Remember that many divers suffer from "I use it so it's the best" syndrome, and will recommend less-than-ideal kit solely because they own it themselves. Never buy anything that's only just come out, unless you like the idea of being an unpaid field tester. A product that's been on the market for years and is recommended by everybody who's used it is a much safer bet than something shiny and untested. Diver and (to a lesser extent) Rodale's magazines do some gear tests that are worth reading.
If you don't know what DIR and Hogarthian diving are, read around until you do. If you know and think it's a bad setup, you don't know nearly enough, read some more. If you think it's good but not for you, you're making an informed decision. If you think it's perfect and everybody in the world should be 100% DIR, please avoid the fora I frequent and stick to DirQuest.
If your instructor/dive club tells you that Nitrox, ponies, twinsets, etc are bad or pointless things, dump them and find some divers who have a clue.
Eliminate clutter - do not take every piece of equipment you own with you on every dive. Take only what you need. A superb rule of thumb is "If you don't need it, don't take it. If you do need it, take two." There are plenty of divers out there who carry two cylinders, two masks, two lights, two depth gauges, two dive timers, etc, and yet still look like they're hardly carrying any kit. Dive shops sell hundreds of gadgets that sound really cool but are utterly worthless.
Eliminate complexity - aim for a setup that requires as little setting up as
possible at the dive site. The less individual items you have to attach during
the kit-up procedure, the better chance you have of remembering everything and
having it all in the right place. Items mounted permanently on your BCD or
stored in pockets cut down on your on-site organizing. It's often possible to
replace buckles with bungee, which is easier to put on and eliminates dangling
straps. eg, YD'er Daz's fin strap, or the WKPP's compass:
Colour: Black may go with everything, but outside of tropical reef diving, it
really doesn't help you be visible. Diving in dark or silty conditions, waiting
for the boat to see and collect you, the last thing you want to do is make
yourself hard to find. Rather than wearing all-black and strapping on flashing
lights to make sure you can be seen, just go for a bright, visible colour
scheme. Red and yellow are the two most visible colours at the surface.
Reflective material is also good: Reflective bungee is available, reflective
strips can be sewn onto hoods, gloves, cambands, even
BCDs if you know what you're doing.
Deficiencies: Never buy a piece of kit to try and overcome a lack of skill. A
dSMB with an inflation bottle might seem like a wonderful way to overcome your
inability to deploy a normal dSMB, but if the bottle isn't full, or there's a
problem, you're stuffed. The inflation bottle should be a convenience you can
do without, not a crutch you're totally reliant on.
Think it through: There are many gadgets that seem like a wonderful concept.
Before buying anything, think seriously about what it does, why you need it,
are there better solutions, etc. If you have a main DV and an octopus, do you
really need an integrated regulator-and-BCD-inflator unit as well? They need
more maintenance, the non-standard button arrangement can cause confusion, and
they're awkward to use. How likely are you to need three DVs
at the same time?
Don't rely on tradition: Many diving practices and equipment configurations
were set up years ago when diving was very different. Crack bottles on BCDs
came into being when there was no such thing as a direct feed inflator.
Quick-release weights were necessary for a wetsuited diver with no BCD, are you sure you need them with your drysuit, BCD, and reel & SMB? A
dive knife is the traditional way of getting out of entanglement. Get yourself
some fishing line or net, tangle your foot around it, and see how easy it is to
cut yourself free with a knife. Compare this with a pair of scissors. Never buy
or use a concept unless you've thought it through objectively and decided it
really is the best way.
Fit for purpose: A tiny reel three inches across might be wonderfully easy to
stow in a pocket. But how convenient is it going to be to reel in thirty meters
of line using such a small spool? How easy is it to wind that dinky handle
wearing thick gloves? How prone to bird-nesting is it? How comfortable is it to
hold onto for the entire dive? And so on. Your priority is how easy and
reliable something is to USE. How comfortable it is to carry when not in use is
a definite secondary issue.
Strobes are another good example: In reduced vis
dives, some divers like to carry a small flashing light, to increase their
visibility. There are two types of strobe: Nice little units that blink on and
off with more than enough light to be visible to other divers; and hideously
annoying repeated-camera-flash units that light up the entire area. These
things are overkill, and dangerous: They destroy the night vision of anyone in
the area, and dazzle anyone looking in your direction.
Bright flashing strobes are meant as markers for shot lines, to enable boats to
see you or your SMB in low visibility (dark or foggy), or for emergency
situations. They should NOT be used on a diver during routine dives. If you
really think you need a very bright light, buy a powerful torch. Apart from
anything else, any other divers in the area may let you know exactly how
unimpressed they are via physical means. If you're lucky, they'll just switch
the things off. (It's happened!)
By being so bright, they force your buddy to NOT look at you, just to prevent
loss of vision. Fine if you just want him to know where you are, but if you're
tangled up or out of air, do you really want your buddy to be deliberately
looking away from you and relying on the peripheral glare of light to ensure
him all is well with you?