[MNWayne]

Be patient. Brewing takes time, but minimize the time requirement by controlling your ferment temp. It's real easy to geek out on equipment and obsess over details. Try to keep it as simple as possible and don't overthink the process. Don't overcomplicate recipes and change only one variable at a time. Take notes. Kegging saves a lot of time over bottling. Learn how/why to adjust your water ions.  Read Brulosophy. Enjoy.

[Bilsch]

I wish I'd known that most all books on home brewing are a waste of money.
And also to not read Brulosophy.

[charles1968]

I wish I'd known that most brewing advice offered on the internet is wrong. And that all grain brewing can be quick, simple, and doesn't need as much equipment as you think. And that simple beer recipes work as well as elaborate ones. And that trying to clone commercial beers is usually a waste of time. And that most process errors will not spoil your beer, as they don't matter very much.

[C Hansson]

I was glad that I listened to John Palmer talking about his five tips för brewers when I was a new brewer. I decided to follow his tips and now when I have everything down my beer is at a level that I'm happy with.

His tips are:

1. Sanitation
2. Fermentation temperature control
3. Pitch enough yeast
4. Develop right recipe proportions
5. Control water

Those five things was what I concentrated on as a new brewer and I think every tip made my beer better.

[HighVoltageMan!]

I wish I'd known that most brewing advice offered on the internet is wrong.

No sh#%!  The loudest mouth usually drowns out the few that have good advice. Beware of what you read or see on the internets.

[charles1968]

I wish I'd known that most brewing advice offered on the internet is wrong.

No sh#%!  The loudest mouth usually drowns out the few that have good advice. Beware of what you read or see on the internets.

One of the great drawbacks of the internet is that anyone can publish anything. Democratisation of information means peer review and quality control are lost. Be careful what you read, it's probably BS.

That why sites like Brulosophy are important - there's an empirical process involved, rather than just opinion.

[klickitat jim]

I wish people could remember what they knew about beer before they knew everything about beer. The lost ability to just simply enjoy a beer without having to dissect it, critique it, argue it's actual style, reduce it to a set of numerical data.

[Wilbur]

I wish people could remember what they knew about beer before they knew everything about beer. The lost ability to just simply enjoy a beer without having to dissect it, critique it, argue it's actual style, reduce it to a set of numerical data.

I agree.

If using tap water (which is just fine)

Have you tasted my tap water? It's chunky with minerals.

[kgs]

I wish I would have skipped extract and started with BIAB. Of course, BIAB didn’t exist then.

PS. I now brew 3 gallon batches. I think you should size your batch based on what you can consume. My wife doesn’t drink and I don’t have friends over a lot to drink. So it’s mostly just me. I take growlers to parties a few times a year.

I brew 3 gallon batches for similar reasons, PLUS, I wish I had known that brewing smaller is physically much easier throughout the brewing process. Moving from 5-gallon to 3-gallon batches was a revelation.

I wish I had skipped my "partial mash" phase and either started with all-grain or gone from extract to all-grain.   I wish I had focused more on temperature control early on, particularly mash temp, post-boil chilling, and fermentation temp.

I mulled over whether I wish I had gone straight to kegging rather than all the bottling I did for six years, but sometimes processes seem easy because you forget what it took to learn how to do them, and kegging is a spendy investment if you're not sure you will be a regular brewer.

[hopfenundmalz]

Everything!

If I were to start over, an electric brewing system would be my choice. A guy in the club has a really nice electric BIAB system.

[TMacphersonNH]

I wish I was aware of BIAB, if it existed when I started brewing. After following what used to be, "the normal progression", I ended up there. Currently working on being able to BIAB during the colder months.

^^^^

This

[Slowbrew]

One simple thing that would have helped me quite a bit, way back in my youth (feeling old today), is knowing a five gallon pot will work for extract brews but a 10 gallon pot will make better beer.

I have two small 5 gallon pots that work well for my two all grain batches a day brewing schedule but my 12 gallon kettle is the thing that makes it all work.  As was pointed out earlier, bigger can be better in some cases.  Don't assume "I'll only ever brew extract and five gallon batches" because you'll likely be wrong.  Get a big kettle if you can afford it.  That will save you buying it twice.

Paul

[EchoValley]

Make sure you're having fun when you brew, it is a hobby after all. 
Brew what YOU like. 
Limited funds: Start small & build up as you can, many a delicious beer has been brewed on a 5 gallon or less gravity setup.
Unlimited funds: Go crazy and be sure to invite the gang over.
What everybody else said about info on the interwebs (except for the AHA site) 
There are no shortcuts to sanitation.
Listen to brewing podcasts that are not produced by 7th grade boys who just learned the F bomb.  There's some old guys out there with good podcasts, probably a couple that even hang out here.
Don't fret over having to dump a beer, it happens.
Take good notes.
You will be your hardest critic and that's fine, because your family and friends will just love all the free beer and tell you it's great.
Brewing software!
Go RO/Distilled or get a water report and Bru'n Water.
Toys and gadgets don't make good beer, attention to detail does.
Read the first sentence again.

-Scott

[santoch]

1) The yeast work on their schedule, not yours.  Don't rush them.  Give them the time they need.
2) Sanitation is King
3) Temp control is Queen
4) Basic water chemistry is the Jack - You don't need a pHD in chemistry, but A) get the chlorine out before brewing and B) make sure you have a reasonable water profile. If your water is crappy, you'll brew crappy beer.  If its loaded with stuff, dilute it with distilled/RO and go from there. If its lacking minerals, add them. 
5) The yeast/fermentation (see 1, 2, & 3) are responsible for far more of the final flavor profile than a newbie can possibly imagine. Try splitting a batch of wort with 2 different yeast strains to get the full effect.  Even related strains cause differences that make them taste like 2 completely different beers.

[dmtaylor]

Here's a couple more:

Carapils is worthless.  It doesn't do what anyone says it does.

There is no magic about corn sugar for priming either.  Use table sugar.  You already have some in your kitchen cupboard, it's cheap and it's effective.  (Keggers need not respond to this either, thanks.)