Though formally hired as Emma's governess, Miss Taylor, or Mrs. Weston as she later becomes, acts as more of an older sister towards her young charge. Among other things, this means that Miss Taylor doesn't adequately step into the shoes of the deceased Mrs. Woodhouse. Unlike her predecessor, discipline's not really Miss Taylor's thing. She's more of a boon companion than an authority figure, and quickly develops an easy intimacy with Emma.
The downside of this, of course, is that Emma loses her way a little. Without an adult authority figure to guide her, Emma becomes quite conceited, thinking herself better than the denizens of Highbury. Miss Taylor's friendship with Emma has had the unfortunate effect that the young lady's numerous character flaws have gone unchallenged, greatly hindering her development as an adult. Miss Taylor's indulgence of her charge has led to a complete role reversal in the normal teacher-student relationship, as Mr. Knightley shrewdly observes:
You might not give Emma such a complete education as your powers would seem to promise; but you were receiving a very good education from her...
Once Mrs. Weston (as Miss Taylor's now become) gets married and has to leave Hartfield, Emma is understandably bereft. Not only has she lost a very close companion, but she knows that her days of getting away with murder are well and truly numbered. Emma must soon put away childish things and start to mature as an adult, a long overdue process that was only further delayed by her former governess's well-meaning indulgence.