I can't think of anything that I really 'regret' buying. I do think that in the long run I will replace my jacket BC. The question between then & now (and this isn't urgent, it's gonna be quite a while) will be do I buy a back inflate BC or a BP&W?
I was suckered into buying 2 of the most expensive everything, including two boxes of stuff I've never used but it sits there just in case, now years later.
I can't say I regret purchasing a BC, but I never could get my bouyancy with it and after losing some lbs it didn't fit me anymore and popped off on a night diving, attached only by the reg in my mouth at 60'. Rather than purchasing a new smaller BC, I went with a BP/w and Hogarthian setup and from the first dive had my bouyancy under control. I have not regretted that. In fact, in a few days I'll be picking up a doubles wing for the same BP from Tobin.
Other things are those yellow flashlights that I quickly replaced with the much more envious LED's my buds were using and now, starting next week, a Salvo HID.
Fog Cutter. Cool factor for sure, but its huge and I prefer a smaller knife on my harness, anyway, not on my ankle. When diving in dense kelp, it is the source of most of my entanglements.
AIR 2.
Console computer. Write mount/wireless is so much better. I don't use wireless anyway, just use an SPG.
strobe lights for tank markers on night dives. I find them annoying. If you can't see my light cannon 100 or canister light, then my blinker won't help my buddy find me any easier.
Neoprene wetsuit. Hated that thing. How many discomfort problems and near panic's it caused from its constriction. After dive 20 I replaced it with a hyperstretc type wetsuit and haven't looked back.
Soft soul booties.
Most of these I didn't "regret" per se, just I didn't know better and I very quickly outgrew them. I'm thankful for the experience but have moved on from them.