A lot of pidgey and those stupid bats around. Interestingly, taking the dog for a walk is not fruitful, a 40 minute 4+km walk will result in me seeing one or two Pokemon.
I think they have the same issue as with Ingress, playing outside of a densely populated area is disproportionately difficult. However, unlike ingress that is based on population control, it seems odd that Pokemon are so rare in parks.
Eta: the app also readily forgets your options (i.e sound off).
Also, has a lot of trouble processing gym battles. Regularly freezes, forgets results and resets mid battle. I suspect it's an issue assosciated with the resources. Yesterday I won my 10 gym wins badge, having never had a battle and without it freezing (whether I'd won or lost), and having never seen a gym change control.