imo there are far too many reasons you can't build more improvements, and few of them are readily apparent when your trying to build.
hills i guess i can see... and its an easy fix since your sov can just get rid of them. but forests? has civilization really fallen so far that we've forgotten how to chop down trees? i spent 14 turns wandering around my starting area one game and kept getting "must build on an open tile" or some such on every single tile, even the ones on the borders of the forest where it didn't look like there were any trees.
As far as not being able to build within 5 tiles of another city, i can understand that for the initial settling of a city, but i think it should be a lower number for improvements, like 2 or 3 tiles.
and the tile limit is frustrating. not because 50 isn't enough. 50 WOULD be enough, if resources within the city walls didn't count. but because if you touch a resource with an improvement it gets walled up, and if that resource is touching another resource, they both get walled up, 50 tiles does not stretch farm enough by half in most cases.
sorry if that sounded ranty, its meant as constructive criticism, i love the game and I've spent nearly every free minute playing since prerelease. just want to see some things improved 
edit: i agree that there does need to be some kind of limit on cities, lest they get way too big, but even lvl 5 cities, while huge, are still only 3000 people. that barely qualifies for its own school district where I'm from. I would like to see the limit moved away from a hard tile limit and to a commerce/upkeep based limit where the more tiles a city occupies, the higher its upkeep. so if you want a huge city, you can do it, but you have to be careful not to let the upkeep exceed the income.
