Transitions are an important part of video editing, but there are a lot of times when it's better to leave well enough alone.
Back and Forth
The advent of non-linear editing software made the possibility of using animated transitions in film projects not only readily possible but easy enough to be a tool of creative experimentation. Now there are enough transition effects in most editing suites that you could probably use a different one for every shot change in a medium length project. This does not end up being a new thing for novices who want to lay down the effects as liberally as possible in an attempt to cover up their lack of finished experience in their project. The opposite ends up being obvious in their final film where excess in transitionary images looks both cheap and oddly distracting. There are a number of times transitions period should be avoided, but it takes a clear vision of both expectations of the audience and purpose of the filmmaker to make those decisions.
The Perfect Fade
Transitions as they were used in linear editing systems where the actual film stock was altered in the process were very basic because the technology was fairly simple. Most of them including fading from one clip to another or fading to black in between. When we look to silent films there is a larger use of things like wipes and irises, but sound films forced a more subtle form of filmmaking and these obvious types of theatrics became archaic to standard, literal filmmaking. As film has progressed there entered a standard for using the fade transitions and this standard persists mainly because the audience is used to it. If they are supposed to be communicated with by the filmmaker there has to be a certain amount of common language adopted by the director. They have to apply some of the conventions so the audience will be able to retain their sense of space and time so that they are better able to stay involved in the film. This being said, there is a standard use for fades and cross-fade transitions. All of these tend to mark a dramatic change in time, place, or both. They should never be used in cutting between different shots in the same scene, and if there is not much time passing by between one scene and another they should be avoided as well. If scenes are essentially a sequence where one leads into the next perfectly the fades will just detract from the process. A fade to black should signify a more dramatic change in time and place, therefore allowing the audience to clear their canvas a little bit. The cross-fade, where one image fades into another, yet this is meant to signify that there was a change but that it was not very dramatic. If you are planning on using fades for any other reason they will confuse the audience, so only use them in counter situations if that is what you intend.
Now that applying film transitions is so easy many software packages pride themselves on how many animated ones they can come up with. This is why we collectively see a huge rise in videos of dance recitals with transitions that make the screen look like a rain of shattering glass or an atomic explosion. Almost all of these should be avoided because they look amateur in their very inception. Even if they are impressive at first, which they usually are only to the most obscure hillbillies wandering down from the mountains, they are not the product of your creativity and will not look as though they were. They will look like an obvious attempt to be impressive. Instead of succeeding at classic film construction it will be distracting and will not appear as though you wanted to attack tradition. These are true almost all the time, unless you are attempting to make the transitions the focus of the audiences’ attention. There is no absolute rule for this, but this is often used to highlight the aesthetic element of a film. For example, the manic Canadian Guy Maddin often uses visuals and music that scream early American silent films, and therefore he often uses obvious transitions from that period. A cube flip or wipe might work well for a jovial scene that has some very upbeat music.
A Drop Will Do It
Like any part of filmmaking you need to have purpose if you are going to stray from story focused design. Anything that draws unnatural attention to itself will take the audience out of the story, and that is usually a bad thing. Keep this logic in place when choosing transitions. Less is always more.