Interesting stuff, this. I find it fascinating how the medium influences the final production in ways that I wouldn't have guessed.
I would say, though, that the only difference between TV and cinema as a *medium* come from the different aspect ratios. There are many culture differences due to the early days (standalone/silent/permanent versus serial/vocal/live/transient [Fiona/Neil]), but as Tavia points out, these culture differences can be avoided.
Instead, I suspect that the differences now are down to the relative industries. The B7 fights look bad not because they weren't given sufficient importance, but because the actors are involved. I was surprised to discover how little support Pruitt and Crawford got for the Buffy fights, but because they're arranged and filmed by professionals first, with a couple of flashes of the actors added later, it all looks much better. As Fiona says, if the budget doesn't stretch to hiring those professionals, then it won't look as convincing.
Production values vary, sometimes with surprising results. On Xena, for example, they don't shoot a scene with multiple cameras. They shoot it several times, moving the camera each time. They do this because they have to move the lighting anyway, to make the other POVs look ok. If you don't have the budget to do all that, you have to compromise on the lighting.
Tavia mentioned reselling productions to other countries. Fine, if you can do it. Neverwhere was shot on video, to save money, but consequently had (according to Gaiman) was far less saleable as a result.
Then there's how the script gets affected. JMS advises having as few people in each scene as possible, because the more people in a scene, the more people to screw up the take. Gerald Kelsey recalls writing a script which had a character who was both chauffeur and gardener, because it would be cheaper to have only the one actor. It wasn't, because the same actor who have had to be retained for longer, so the two roles were split between two actors. In another script, he had two principle characters on bicycles, with dialogue. The second-unit director shot two other actors from afar, and had the real actors add a voice-over, because that was cheaper. And, happily, gives a wide, sweeping shot, too. :-)
steve