I am planning to use the boost feature for a horde toon. Having all alliance toons I dread the thought of leveling up a horde. I might miss a little story but not enough for me to overcome the dread associated with leveling up from the start. Plus it's kinda lonely leveling up as I found recently with my panda toon.
What i understand from the information blizzard shared, is boosted toons will get a basic set of level appropriate gear which will replace any existing gear. existing gear is supposedly mailed to you if it was equipped. for players without existing level 90 toons, boosted toons will have a limited set of abilities with new ones unlocking through a short introduction. for players with existing level 90 toons, it's onward and upward straight away. likely this will change dozens of times before release anyway so take it with a big grain of salt.
As to learning how to play a class or spec, I like the direction Blizzard is taking with proving grounds. This is long overdue and hopefully leads to a successful feature. Is it going to be perfect? No but seeing the effort to address this problem is encouraging. As I noted leveling up recently was very lonely, would go hours and hours across multiple zones without seeing another player. The leveling experience today is radically different from the release of the game or even the first couple of years so something like proving grounds is needed to help new players learn abilities, roles, specs and classes. While proving grounds are currently just max level, should be possible to use similar technology for new players to run low level dungeons or group quests while learning to play the game.