Been thinking more on this topic as it is obviously right in my professional wheelhouse. A lot of the changes to fantasy as a genre have to be understood in the context of massive changes to how people entertain themselves. Howard was being published at a time when reading really was mass entertainment in a way that is difficult to appreciate today. This dated back to the rise of serial publications in the 1800s, and culminated in the pulp publishing industry of the 20s and 30s.
The result was a huge demand for material to publish, and genre fiction was particularly popular with, and targeted at, in literary terms, a less educated and sophisticated audience. Underpaid, overworked writers really were churning out potboiler material, a lot of it of dreadful quality (go review any particular issue of, say, Amazing Stories, etc. from back then). This led to genre fiction being widely disparaged amongst the literati, and usually not without reason.
However, as other entertainment options have flourished, reading has become a more specialized form of entertainment. And a lot of kids who grew up on those pulpy stories kept that love for the genre as they became older and more sophisticated readers and, occasionally, writers. So there has been a lot of blurring of the lines between genres, and writing genre fiction is no longer as disreputable as it once was (there are exceptions; one of my former writing students is a best selling adult romance author, and she still publishes under an assumed name).
Consequently, the boundaries of fantasy have greatly expanded and blurred since Howard's time, and IMO the quality of the writing is generally much higher.