What I found is that the width of the river has to be very narrow (which usually means not too deep as well). In SimCity, if the elevation is too close to "sea level" it won't place a bridge - gives you a message something like "unsuitable terrain". Also, if the elevation is low, but not quite sea level, it will build the bridge but create a hill for the landing. Often when this hill is imported into the neighborhood, the bridge and the road won't appear to be connected. (See the "deal breaker" picture in the second terrain I posted in the Shared Neighborhood thread
http://www.moreawesomethanyou.com/smf/index.php?topic=1400.0 it's the post where I say "bridges are a pain in the ass" ). But, if the level is very low (almost like beach terrain) and the water level isn't deep (water barely visible in SimCity) once you go into the Sim neighborhood, you will see that the water is bluer and deeper and you can use the Sim neighborhood bridge.
Other things I noticed:
You tend to have more difficulty placing the bridge if the elevation is different on the two sides of the river. This is especially true if you are trying to place the Sims neighborhood bridge.
Try to have a stretch (a few tiles) of straight road after the bridge connects across the river. In SimCity, I had a cross road right near where the bridge hit the river bank. Looked fine in SimCity. In the Sim neighborhood, instead of there being an intersection, the cross road was actually running straight thru the base of the bridge.
When looking at your river (especially when trying to make a very shallow one) look closely and see if the water is "flowing". If there isn't a flowing animation, somewhere along the river the elevation isn't low enough and when you import it into Sims, that point will be much more visible and look funny.
Other SimCity stuff:
If you place two roads right next to each other, even though they are right next to each other in SimCity, when it is imported into Sims, there will be a strip of tiles between the two roads (one tile wide). I use this to make boulevards and major roads with "park islands".
Any road going up a hill or mountain that looks like it has supports (similiar to a bridge) will not show up in the neighborhood. Neither will tunnels.
I'm not at home right now, but I think I remember each tile in SimCity is three tiles in the Sim nieghborhood. So, in SimCity if you have two roads with two tiles in between them, in the Sim neighborhood it will be six tiles wide.
For some reason, it seems to import the terrain's mirror. So, if you have a lake on the right hand side, when you import it, the lake will be on the left.
If there's anything else I can help you, with let me know.