In addition to the cleanliness of the lab, Walter also decided to cut the pseudoephedrine from over the counter drugs out, along with the steps required to extract it from the other ingredients in the cold medicine and instead synthesised his own from methlemine.

No idea which one is easier to get a pure result from, but that was his first "level up" to the blue meth he became known for.

Though what set him and Jessie apart from Gail is hard to say, other than Walter sabotaging Gail instead of just teaching him his techniques (iirc, been a while since I saw it).

The series does seem to be taking the stance that "before Walter White, professional chemists worked for legitimate companies doing legitimate work in professional labs while drugs were made by hobbiest chemists in ad-hoc labs that were neglected even if they had professional equipment and the people running them had no idea about the single most important factor when targeting high purity: the purity of everything else along every step of the process or some way to extract purity before the end step".