It's too easy to find Pokemon. Currently the game is mainly about collecting all the Pokemon and once you have majority of them there it not much left to do.
Any Pokemon can be found in just about any area given enough time, this kills the exploring concept. Non-common Pokemon should only be found in different geographic areas taking into account thinks like climate, water sources, forested vs plains vs desert.
Too many Pokestops that are way too close together making it easy to walk in small circles and again limiting the need to explore other areas.
Unfair advantage for people in major cities vs normal cities vs rural areas.
Pokestops and pokemon seem to be clustered in downtown areas.
Walking trails in many parks (which do show up as trails on the map) have nothing which I found to be very disappointing and again limits the need to explore.
The game is way too easy for people to play using "drive by and pull over" techniques.
Cheating got way out of hand. Gyms retaken minutes after I took them with no other person in visible site and all the other cheating.
Too easy to level up from 1-20.
Stops resetting after 5 minutes reduce the need to explore and continuously provide items and xp, they should only reset once a day.
All that said, any new game is going to have it's initial spike, drop, and level off with a slow decline and smaller spikes as new content is released. This decline should not be unexpected. However the company could have done a better job to take advantage of the initial spike so it last as long as possible.