Hmm. All right.
So, the thing about trading, as you note, is that it broadens a Pokémon’s view of the world. Being with different trainers puts Pokémon in a wider variety of situations and exposes them to different styles of training. In the process they learn greater independence and versatility, gain additional perspectives on their powers, and work together with a wider group of other Pokémon. Basically, traded Pokémon get all the things Pokémon could possibly want out of having relationships with humans, only they get more of them, hence the boosted experience deal (the larger boost for Pokémon received from a person who speaks a different language is a natural extension of this, representing the effect of a Pokémon being transplanted not just to a new trainer but to an entirely different cultural context).
On the flip side, trading a Pokémon most likely means abandoning your relationship with it and handing over the reins to someone else. Taken in the context of my theories about what it means to “capture” a Pokémon – namely, that Pokémon cannot be captured unless they are at least open to the idea of being trained by humans, and that the act of capturing a Pokémon represents the formation of a sort of implied contract – this is potentially not a totally legit thing to do. Your Pokémon challenged you, tested your worth, and permitted you to become its master. Now you’re letting this other douchebag take over? What the hell, man? Of course, potentially a Pokémon might accept that you’ve made a good choice and go along with it, enjoying all the benefits I outlined above. It’s entirely possible, though, that a less powerful trainer will be found wanting – and this may be true even if the new trainer is actually higher in formal rank than the original one! After all, the original trainer is still the one the Pokémon tested and accepted.
As I suggested in the entry on Battle Aboard the St. Anne, the way you as a trainer feel about this says a lot about your relationship with your Pokémon. It’s oversimplifying things to say that being willing to trade means you don’t care about them, because there are obvious benefits. It does, however, imply a very different point of view: trainers who are prepared to trade their Pokémon around at the drop of a hat are likely to have a more condescending view of Pokémon as a whole – they may very well be concerned with doing whatever is best for their Pokémon, but are very sure that they know what that is better than the Pokémon themselves do.
Trade evolution, of course, is the other big thing. Here, Black and White have done something that annoys me a little. See, prior to Black and White, I could say quite easily that the idea of Pokémon such as Haunter and Kadabra evolving when traded works as a natural extension of the way evolution is typically presented in the anime – as a process closely tied to a Pokémon’s psychological development. Evolution is triggered by all of the beneficial effects of trading that are also responsible for the experience boost. The problem with Black and White is that they introduce Shelmet and Karrablast. The Pokédex makes it clear that these two evolve when they are “bathed in an electric-like energy” together. The implication is that their dual evolution – and, by extension, perhaps other trade evolutions – is triggered by some aspect of the trading process itself (which, in both the games and the anime, is facilitated by a needlessly complicated machine). Now, personally I think that Karrablast and Shelmet’s dual evolution, while a brilliant concept, could have been much better-handled all around, so I’m going to suggest, firstly, that they’re an exception to the general rules for trade evolutions, and secondly, that there are actually many processes and phenomena which can cause Shelmet and Karrablast to evolve, trading being only the simplest and (for the majority of trainers) the most accessible.
I don’t believe my theories about trading are by any means comprehensive, but I think they should at least stand up to casual scrutiny. If you can think of any more specific examples that might support or refute anything I’ve said (particularly from the Hoenn and Sinnoh series of the anime, or the manga, which I’m not familiar with) I’d be happy to give that some thought.