In my opinion, the Islah party is the one to watch when it come to reform emanating from the "opposition." This is because of their dominance within the Joint Meeting Parties, as well as the fact that they are the most conservative political party within that grouping (thus, the most likely to acquiesce to the regime).
As Paul Dresch in "A History of Modern Yemen," and Ahmed Hezam Al-Yemeni in "The Dynamic of Democratisation – Political Parties in Yemen" both explain, at the party's foundation in 1990 it was made up of intellectual‐reformist Islamists (or the Yemeni Muslim Brotherhood, personified by moderates such as Abdul Wahab al‐Anisi), a conservative tribal group (led by Sheikh Abdullah al-Ahmar), and a conservative‐radical salafi group (best personified by the infamous Sheikh Abd al‐Majid al‐Zindani).
With the recent defection of al-Zindani from the regime - albeit, his desire is to found an Islamic state - it looks like Saleh's authority is further eroding. Coupled with the almost complete defection of the al-Ahmar's, it will be interesting to see what reformist Islamists like al-Anisi and Mohammed Qahtan (a co-founder of Islah along with Zindani and Ahmar) decide to do in the near future.
Recently, Qahtan was quoted as saying "We support the people, but we wish they had waited a little bit to protest so much... the reform movement in Yemen requires wise deliberation. We need to be careful against taking an impulsive course of action, or there will be losses." I'm a bit sceptical of wise deliberation, as the deliberation in Yemen - be it a facade or not - has been going on for far too long.