I started with a good handle on the importance of temperature control (although my techniques for achieving it are improving in terms of reducing the amount of effort to maintain it), but from what I've read (not counting this thread), this is a "must" for producing repeatable results. My first beer was a little weak, but other than that and one late-onset infection, I haven't had any of the typical "homebrewer" flavor problems. So if you're not already controlling the temperature, at least to prevent going over the recommended temperature for your yeast, I would put a very high priority on that.
In terms of changes I've made that improved things the most, I think going to partial mashes has been it. I did it pretty early, so it's hard to separate the effect from just getting more familiar with the process, but it has opened a lot of doors to recipes that might be hard to pull off with extract alone.
My wort chiller made a big difference, particularly with respect to safety, but the OP already has that so no need urging them to pick one up. But if you don't have one, I feel a lot better being able to leave my kettle on the stove and chill it instead of having to trust the handles to keep gallons of scalding, stick wort from spilling all over me.