I was reading through twitter (like you do) when I came across an interesting conversation involving Jeremy "Muffinus" Feasel - he asked the question how long is too long for an expansion intro? Well, he asked it better than that.
Cataclysm and Mists of Pandaria continued this to some extent - if you were a level 80 character, you didn't get much more introduction than 'bad stuff is happening, you can go here or here' but the two zone options, Hyjal and Vashj'ir, were very detailed and had extended sections of on-rails questing. Cataclysm also had two complete starting zones for worgen and goblins that served as introductions to the meat of the expansion. Mists had the pandaren starting zone, but it also had the Jade Forest which had an outright introductory feel that was a lot stronger than any zone introduction had ever been, combining elements of the DK starting zone and the Vashj'ir start.
Because Warlords will not be introducing a new class or race, but will have the option of boosting a character to 90 to immediately start on the new content, some sort of actual introduction to the expansion content definitely seems warranted. However, I do agree with what a lot of people tweeted to Muffinus - once you've seen that kind of introduction, it gets wearying to see it again and again on each new character you bring through. I know I haven't rolled a new goblin in quite some time, because while I know that I can skip most of Mulgore and head elsewhere if I want to on a tauren, I'm stuck in the goblin starting experience for fifteen or so levels no matter what I do. It's not that it isn't a great experience -it's possibly one of the best ever designed for WoW. It's that I've seen it.
WoW is generally full of content we've all seen, of course. The trick is always in allowing us enough of an illusion of choice that we can bear the sameness - and one of the ways classic WoW worked well for this was that there were options, even though they might seriously cost you. I leveled my first human, way back in 2004, entirely through the night elf starting zones even though it meant running through higher level zones like the Wetlands to do it. Giving people an option, even a very difficult one, can cut back on the feeling of sameness.
So what's the answer here? Well, there have been several good suggestions - letting people skip the introduction experience if they've already done it on that account, making the experience a solo scenario to cut back on the issues that plagued the Mists opening (anyone who did the Alliance side experience remembers bottlenecking on the gyrocopters because they glitched out and couldn't tag mobs), or perhaps making it a skippable cinematic. I don't know if I think a quick skippable cinematic is the way to go, although I'd like it if it had a few cinematics the way Jade Forest did (I still love that Sha rising from the wreckage of the Jade Serpent's statue).
For myself, I think about an hour is about as long as it should be before it gets you started on open world questing. The Jade Forest, once the bugs were worked out, was pretty much the ideal length, setting you up with some quests and getting the intro story in your hands fairly quickly. That would be my ideal length of time for being on rails. I also definitely think making some of it skippable if you've already done it is an interesting idea.
That said, I don't think encouraging people to skip these kinds of things is really a great idea. Skipping the Jade Forest intro quests would really leave you lacking a certain insight into how each faction views what it is doing in Pandaria and why. So I'm only really in favor for it being skippable at all by people who've already done it - because in general a lot of really hard work goes into these zones (heck, the Jade Forest was completely rebuilt before launch) and it's worthwhile to see it.