as for the Spren, if you look at the back of the book there's a table describing the 10 virtues, they're associated spren, and the kind of power that comes with it. There's probably a very good reason Vorinism pushes people to choose a virtue and excel at it since it was more likely to attract a spren and therefore towards becoming a Radiant.
I'm going to guess that when the Radiants betrayed everyone, the Spren stopped bonding with people (or the Spren gave up on the Radiants causing them to lash out and betray everyone?). Either way, net result, no more radiants. Spren seem to become more aware the longer they become attached to a person (as seen by Syl) and then go back to being mindless after that.
This actually seems to be the best thing anyone's said yet.
In regards to planet hopping -- How come nobody is freaking out (like I did) when I read Shallan went to Cosmere?
Don't you think that'll be how people start planet hopping? I think it definitely has to relate in some way.
I really like the Spren Abandonment idea, that's great.
As for Shadesmar, (not the Cosmere o_O) Brandon has talked about Hoid knowing some tricks to do with it that allow him to access to all sorts of places, like for instance perhaps the Well of Ascension, but I don't think there's any hints in any of the books as to exactly how he travels between planets.
Now, what do we know of Shallan that would make attract these spren to her? She has a shardblade, is (or rather, was) living a lie, and she murdered her father. Unfortunately, this doesn't tell us much except perhaps the murder. Maybe these spren are attracted to killing? Unlikely, as there would be many Alethi soldiers with spren hovering around. Perhaps these spren are attracted to something else? Deep, emotional scarring? Unfortunately, we don't really know enough. I can't make heads or tails of it, really. What do you guys think?
Well, we don't know what attracts these spren, but perhaps we can find out what these spren are. There are three options that stand out, Truthspren (note how they asked for one truth to enter Shadesmar), Soulspren (they allow you to soulcast, they have something to do with death.) or Shadespren (they give you access to Shadesmar, and they aren't really visible in the physical realm, they are only visible through the cognitive realm).
Can you guys make some sense of this? I've about run out of ideas here.
I think Kaladin actually makes it incredibly clear how Spren are attracted: You exemplify the virtue the Spren represent. Honorspren bind things, and Kaladin bound together groups of soldiers and slaves, protecting them and treating them better than they expected. I imagine Shallan's spren are something to do with intellect, or perhaps honesty, given that they require personal truths to admit her to Shadesmar, and that she started being more honest to herself at the end of her plotline when she first discovered her powers.
Oh, and on shardblades glowing- do you think perhaps that they can be imbued with Stormlight when a Radiant uses a Shardblade?