erecting a spiritual maakeh

The gemara (Kesubos 77b) writes that when R' Chanina bar Papa died, there was a pillar of fire that separated his holy body from everyone:

כי נח נפשיה אפסיק ליה עמודא דנורא בין דידיה לעלמא וגמירי דלא מפסיק עמודא דנורא אלא לחד בדרא או לתרין בדרא
קרב לגביה רבי אלכסנדרי אמר עשה בשביל כבוד חכמים לא אשגח עשה בשביל כבוד אביך לא אשגח עשה בשביל כבוד עצמך איסתלק אמר אביי לאפוקי ממאן דלא קיים (אפילו אות אחת) אמר ליה רב אדא בר מתנא לאפוקי ממר דלא אית ליה מעקה לאיגריה ולא היא מיהוה הוה וההיא שעתא הוא דשדייה זיקא

Abayei said the purpose of that pillar is to keep away those who have not fulfilled the Torah in its entirety.  Rav Ada bar Masna then criticized Abayei, saying that he, Abayei, failed his own test, as his roof was missing a maakeh.  (The gemara then concludes that Abayei had built a maakeh but it had blown down at that moment.)

The point of R' Ada is that nobody's perfect, and Abayei's lack of maakeh (whether intentional or not) served as an example to bring home the point. 

Of all the mitzvos Abayei might be found wanting in that would make R Ada's point, why was it maakeh in particular where he got tripped up?

The Tiferes Yosef, the Radziner, quotes from that Ishbitzer that maakeh is not just about erecting a physical barrier to prevent people from falling off your roof.  It's also about preventing people from falling in ruchniyus.  "Ki yipol ha'nofel" -- there are unfortunately a lot of people who are "noflim."  Let it not happen because of you, because your standards are not those that anyone can meet or relate to.

Rav Ada was telling Abayei that if the standard is perfection, then in your world everyone is a "nofel" and no one will every measure up.  Your standard fails the litmus test of the mitzvah of maakeh.

Of course one has to be careful not to err in the other direction either.  Having too loose standards, ignoring the need "l'migdar milsa" or avoid "mar'is ayin," can lead to equally destructive results.