The use of onion gravy for the bulk of a curry is clearly the largest factor, but i know for sure that blindly following a recipe on here or elsewhere, will unlikely produce the bir flavour at home. I notice how many people new to making curries with base gravy, frequently report "better than their local" results, after only a few attempts, while grizzled curry veterans disagree.
. Something wrong there.
I notice you say that a lot about new members, must be 3-4 times in the past week or so you've repeated that.
I've read many of the long threads here, been a lurker for 2 years and to me it seems a mixture of some "experienced" people have cracked it and some new members have while some new members report to not getting it right, wasting their money and never posting again disappointed while being encouraged to stick at it.
Some of mine lately have been lacking the aroma, depth of flavour. I thought my spices needed replacing but last night my curry turned out as good as the best ones. I seemed to go down a route of over doing the cooking. I changed a bit to keep it more liquidy throughout the cooking, not to dwell to much at each stage and really brought out the aroma and knew soon as I go that smell the curry would be good. Seems to be a fine line between spoiling the curry. I can see more how when chefs talk about preventing the spices from burning, over do it a fraction its gone.
Maybe some so called experienced are stuck on a way of doing things from years ago when less info was around, or can't actually cook, or don't see what is going on in front of them and know what to adjust on the fly. What is experienced anyway? Someone who cooks in general who maybe considers themselves good could easily not get the spices thing for years as its a lot different. Even more so when you get guys who don't cook at all before their curry quest but have been at it for years and seen as some curry guru.
One experienced member was claiming (can't remember the name) how he wasn't getting the bir taste/aroma. Many pages later in the discussion back and forth he actually elaborated that actually he was on about some mystical 70s taste and aroma and could replicate BIR/TA of today. I also read many new members who state they're trying something and never report back. So many dead end threads and posts on this forum. Then you have member jb has replicated everything, had lessons, does the spiced oil properly. Is he not experienced enough?
When I think of good curry houses, the chefs have been brought up cooking spices every day, they build up a knowledge and get taught from parents and know what's happening in the process. Your typical English curry quester hasn't and may never get it right most of the time, conversely he/she may find a way that works by chance and stick to it or is good at observing and taking in info to get better.
Reminds of italians mama's special sauce, you can spend years and never get close. Also when watching some Asian cooking programs, the elder parent said how it took 20-30 years to get as good as their parents.