According to this thread at Apple, this is a bug. This means it will hopefully be fixed in a coming update to OS X.
In the meantime, you have to relaunch the Finder, which you can do either through force quit (⎇⌘⎋, or through the menu), or by less violent means by quitting it normally, which you must first activate through the following command issued in the Terminal:
defaults write com.apple.Finder QuitMenuItem 1
Photostream will store the previous 30 days worth of photos automatically. That means, every photo you take on your iOS devices will be sent to Photostream, but only the last 30 days worth of photos will be available there.
If you enable 'Automatic Import' of Photostream in iPhoto (Preferences > Photostream > Automatic Import) then when you launch iPhoto, it will automatically download a copy of any photos that are in your Photostream, and automatically create an Event titled: {MMM YYYY} Photo Stream (eg. "Jul 2012 Photo Stream")
This Event will be automatically kept up to date, as long as iPhoto is open long enough to sync everything required (depends on your connection).
In effect, as long as you open iPhoto once every 30 days and leave it open long enough for your Photostream to be imported, you should never have to manually import a photo from your iOS device at all.
As for removing items from your Camera Roll on your iOS device, as long as you see that they are present in Photostream (or in iPhoto), then it is safe to delete them from your device. (There is no way around this but to do it manually as far as I know.)
Currently, Photostream does not support videos. To import videos, connect your iOS device to iPhoto and import these manually.
As a side note, in order to keep some photos on the device, you can create a smart album in iPhoto along the lines of "Photos taken within the last 30 days" or "Photos rated 4 stars or more", and configure iTunes to sync these to your device. That way, you can be sure that no matter if you clear your Camera Roll on your device, or delete certain photos, you maintain some convenience.
Best Answer
This could be two things.
The video could be using a codec that iPhoto doesn't recognise. Unfortunately Apple don't seem to publish a list of video codec that they support. Even Apple's listing of image file formats is vague.
The video could be using a recognised format but the file could be corrupt.
I would try opening the file in something that supports a wide range of video formats, like VLC. If the file doesn't open at all, there is a good chance it is corrupt. If it does open, you can check the file format with CMD + I.