Dog poop on the sidewalks. Day two of my stay had an unfortunate land mine incident. However, there is also a fair bit of littering, but unlike North America, very little broken glass. Deposits still mean something. Take the bus out, or walk out to Mount Hymettos and visit the Kessariani Monastery.
Hymettean honey is awesome with all the wild flower tastes.
Parks are used for urination. If you smell a latrine smell walking through a park, or by some bushes, both animals and people have been using it to go. Very user friendly - something that public transportation needs to improve upon in North America. Customer service has not been invented yet. Service with a scowl.