Have been doing extract kits for abt 9 months now. IPA's and stouts. Some turned out OK (but not great), and some had that nasty homebrew "twang" to their taste. Believe I have gotten it down now. Nothing but success with the last efforts. Going back to the beginning of this thread, I agree, there are a few things the kit instructions don't tell you that are, in my mind, critical to success. In my experience, it came down to two major topics throughout the process: Temperature and Aeration. Again, this is my experience with ales and stouts.
Temp Control:
1) During steeping, keep the temp below 165. I now stay at 160.
2) Cool as rapidly as possible after the boil. A chiller is a great investment.
3) Don't pour out of the boiling pot much above 80F. At this stage you don't want to change the chemistry or molecular structure of the wort, and you risk doing so if you pour it out into the fermentor at higher temps.
4) After you pour your ~80F wort into the fermenting bucket and start adding water, stir to keep the temp constant, and don't let it get below 65F. Remember also that you're attempting to strike two objectives: temp (above 64F) and target OG. If you have to err on one side or the other, I've found that temp is the more critical. Don't drop below 65F in order to hit a target OG. In the end, taste is more important than the ABV.
5) Ales and stouts need to ferment in the upper 60's. Don't let the temp INSIDE the ferementor get above 72F. Just because you have your fermentor in a 68-degree basement doesn't mean that you're fermenting at the right temp. Remember that this is an exothermic reaction. I've seen it take the temp inside a 5 gal fermentor more than 6 or 7 degrees above ambient. I finally caved in and bought a ~$150 chest freezer and a Control Products dual HV controller ($68 from Amazon). Best investment I ever made in this hobby. If you can't do that, make sure you move your fermentor to a place where the temp of the liquid stays below 72.
6) After fermentation stops, and you rack to the secondary, they say that temp control is far less critical. True, but on the Low side only. OK to get down into the low 60's or even high 50's, but STILL don't let it climb above ~71 or you're inviting that nasty taste again.
7) After priming and bottling, keep the bottles between 68 and 71 degrees to carbonate. If you let them get much lower in temp, carbonation can take much longer. If you let them get up into the mid to upper 70's, you again risk adverse effects on your flavor.
Aeration:
1) After you've added your cooling water following the boil, hit your OG and are at about 66-67 or so, it is very important to aerate. I've seen references to using airstones. Never tried that approach but am sure it works. I simply make sure my bottling bucket is also sterilized, and I pour the wort back and forth between the fermenting bucket and bottling bucket twice (four pours) until there is a good froth on top. At this point, I pitch my yeast. Most often I simply sprikle dry S05 on top of the froth without mixing it in. Other techniqes (with liquid yeast, e.g.) are also fine.
2) When racking to secondary, and most important, when transferring to the bottling bucket prior to bottling, aeration is your enemy. Again, they don't tell you this in most kit instructions. Boil your priming sugar solution, let it cool a bit, pour it into the bottom of your bottling bucket and then siphon (do not pour) your beer into the bottling bucket with no splashing. Stir the priming solution in VERY slowly so as not to aerate at all. Then proceed with bottling ASAP.
I'm no expert, but since I made these adjustments, my brews have been top notch. Good Luck!