Yemen. Source of photo: Reuters.
1. There are no fake political leaders in exile in the Yemeni opposition.
The political leaders of the Houthis who are fighting the status quo in Yemen are actually on the ground alongside the fighters. In the case of Syria, the official, mainline, made-for-tv political representatives are labelled the "Istanbul Hotel Opposition" or something like that by regime-supporters in Syria because they're too busy chilling in hotels in Turkey to engage in talks with the Syrian government.
They don't have any decision-making powers over the fighters, both local and foreign. They don't have any ideological or political control over the A-Z list of Jihadist terrorist groups fighting the Syrian government and the Syrian people. Maybe their plan is to fly in when all the dust settles, after the armies of both sides have been decimated, and try to act like they've been leading the whole time. But they don't have any credibility in Syria at this point to command anyone's attention.
2. The Houthis have put dialogue on the table, unlike the Syrian opposition. They are making pragmatic deals with local, like-minded revolutionary allies, regardless of sect or political party, as well as old political foes.
Not only have the Houthis made deals with alienated Sunni tribes and other anti-status quo political factions in Yemen, they've also made deals with their former number one nemesis in Yemen, the former dictator Saleh. The Syrian Opposition runs from the very word dialogue and encourages the destruction of Syrian infrastructure and economy by issuing ultimatums.
3. The Houthis are not alienating the local population with stupid, sectarian battlefield massacres and civilian bombings.
There were many reports after the Houthis took the capital of Yemen that they fired on protesters, so these guys obviously aren't peaceniks. Their ascendancy spells overall trouble for the region. But unlike the stupid Wahhabis that have infested the Syrian opposition, they're not going around bombing believers in churches and mosques because they consider them to be infidels. These guys are working with brains and have battlefield courage. The ISIS terrorists have neither.
The Houthis have not taken action to deliberately alienate Sunnis, but that hasn't stop a lot of Sunnis from badmouthing them because they've been instinctively anti-Shiite throughout their history. No matter how much they try to separate sectarianism from their revolutionary political aspirations and try to address Sunni grievances the Houthis will always have Sunni enemies who will never change their minds about Shiites. There's nothing they can do to change that.
4. The Houthis are politically independent.
How many times have we seen Syrian opposition figures on camera with official ministers and leading political figures from Europe, the US, and Turkey? Too many to count. If they had their own agenda, with real democratic principles and a popular base in Syria that supported them with their feet then they've might gotten somewhere by now. But they have no popular support inside Syria so they have to rely on regional and international countries for money, weapons, training, media propaganda, etc.
In Yemen, the opposition has been ignored by the world press for over a decade now. They had no choice but to build from within. The Houthis were busy building a coalition from the ground up while the world turned a blind eye. They make their own political decisions because they have the power to do so. They don't have to import foreign fighters from 80 countries like the Syrian opposition. They have a solid support base, but they're not powerful enough to control all of Yemen so they see no problem with coming to agreements with other groups to advance their own agenda.
5. The Yemeni state is much weaker than the Syrian state.
There is not much to add here. The biggest proof of this is that the so-called legitimate President Hadi ran away to Saudi Arabia from Aden after first running away from Sanna to Aden. Maybe next he'll run away to Britain. Assad, meanwhile, has staked his claim in Syria. His government has lost territory, and the country's borders have been infiltrated from every side, but at least it is still standing.