Sorry for the wall of text, but been giving this a lot of thought.
I've spectated a couple games already, and all in all, have to say that this is a wonderful systems so far. I just have some minor quips and quibbles about it. I have a couple thoughts on things that would be good additions, imho.
First, it seems that there's no way to message anyone when you're spectating a game, unlike a normal game. This simple bit of chat functionality (possibly togglable) would be a lovely addition, in my opinion.
Second, along the same likes, there's options to spectate your friends, but as of yet, no option to spectate the game that your friends are -watching-. That would be an awesome way, with the above chat function, to promote a tighter-knit community, and would also be really handy for ranked teams to study the same match as their fellow teammates were watching.
Third, I'd love if there was an option to pull up (if need be over the screen and he action) the runes, masteries, etc. for the particular player being watched. I think that would be an invaluable teaching tool.
My last few things have to do with expanding or tweaking the current mode.
I think the directed camera could probably use some tweaking, maybe geared towards centering around large groupings of champions. There have been several times where the camera has been cheerfully following a champ farming, running up a lane, or shopping while I watch the little faces on the mini-map converge in the jungle. Also, if there's any way to keep it from getting distracted when there's an engagement. Several times it will show the start of an engagement, switch to another one, and snap back just in time for the kill.
The playback functionality is solid for what you -can- watch. I'd really like some server-side support, though, for the first few minutes of a match. I'm hoping this is going to be a future change that rolls out, because especially for the featured matches, it would be nice to be able to rewind to the beginning minutes of the game to find out why exactly that Naut is 3/0/5 and that poor WW has died 10 times.
Along similar lines, was watching a match earlier and let the directed do it's thing, but I was wanting to go back and watch several points from particular characters' perspectives. Got up to do a couple things, and when I got back, the game had been won, and my only option was hitting "continue" and logging out of the game, losing the stream. If you could continue to have the ability to rewind, and rewatch, even after the nexus explodes, that would be another really cool bit of functionality. I'd love to watch a match as it unfolds, and then hop back over the shoulder of my favorite character, or someone I noticed did very well.
tl;dr Basically, it's a good system so far, that needs more server-side recording support, and a few new features to support your friends and the community at large.