The announcement of patch 7.1 before Legion has even launched is a cause for much excitement. Not only is the centerpiece of the patch a return to one of WoW's most beloved instances (in a way that does not get rid of the old version of said instance,) but it also implies that Blizzard has planed out this expansion to have pacing similar to what we saw in Mists of Pandaria, where there was always something to look forward to (well, until Siege of Orgrimmar went on for fourteen months.)
Warlords of Draenor's biggest failing, in my opinion (and I think most peoples') was that there was so little to it. Given that 6.1 was a nonentity, the expansion really only had one major content patch in 6.2's Fury of Hellfire. 6.2 added a decent amount of content (though the naval missions left much to be desired) but as the sole content patch after Warlords' launch, it was pretty damn disappointing. One of the apparent causes of that was that Warlords was intended to last only a year. Blizzard had been talking for a long time about getting expansions out at a faster pace, but thankfully Warlords taught them that A: even if they try to do that, they won't be able to and B: a "faster expansion" means about half as much content, at which point your sort of wonder what the point of releasing the expansions faster is (other than the price of the expansion itself - but WoW has always made way more of its money through subscriptions, and so it behooves them more to make sure people want to stay subscribed consistently rather than shelling out frequently for expansions - and players certainly would prefer it that way as well.)
Now, does that mean we won't have a content drought after Legion's final raid arrives? That's what they claim, but I've also been to this rodeo many times and I remember that even Wrath went for roughly nine or ten months with nothing new (except the Ruby Sanctum.) At some point, the WoW team needs to switch from patch development to the next expansion, and that's always going to take more time to complete.
That said, you could imagine an alternate history where Mists of Pandaria spread its patches out a little farther. If they had taken one extra month between patches (and they were pretty fast-paced, so I don't think it would have harmed them too much to do so,) the Siege of Orgrimmar span would have been reduced by four months - still a bit long at ten months, but clearly not as bad.
Blizzard has stated that they intend Legion to be a longer expansion than previous ones - a complete reversal of the disastrous "faster expansions" strategy. Whether that means it's intended to go on for two years, like all expansions have wound up being in practice, or if they want to have it last even longer is anyone's guess.
Personally, my hope is that we're going to see more than three raid tiers. The only expansion to ever do this was Wrath (and to be fair, tier 9 was kind of a mini-tier.) They've also made it clear that they will be adding more dungeon content as the expansion goes on - Karazhan's new version will be the first post-expansion-launch dungeon since the end of Cataclysm.
There are also plenty of reasons we might want to stick around in this expansion - the class order halls and artifact weapons both seem very expandable. I have to imagine there will be a lot of new artifact appearances as the patches roll out. Plus, given that we're fighting arguably the most important villain in the Warcraft universe, it ought to feel epic, and doing a slow build really helps with that.
So even if I'm eager to get some hypothetical Old God underground expansion (I've been DMing my first D&D game and I'm really eager to get the players back into my setting's equivalent of the Underdark,) I think it could be great for the game to have a massive expansion with tons of patches spreading out over a long period of time. Hopefully that's what Legion will be (and with Demon Hunters in the game now, I'm less eager to rush to the next expansion for a new race or class.)