Reuters has a story, Pope's Latin mass plans spark concern which probably gives a good succinct presentation, by means of quotations, to the opposition with regards to the Motu Proprio.
Effectively, the concern, at least as it is presented here, is not the liturgical rite itself (though I think this point is debatable, for while it may not solely be about the liturgical rite, it would seem that it is most certainly a part of it, which is witnessable in the opposition as well to a reform of the liturgical reform) but rather some of the worldview, if you will, of the SSPX and such.
Indeed, if we're talking about Bishop Williamson's worldview, this is one thing and who of any Catholic sense can't but agree that it is highly problematic and undesireable? But I think there is need for a sober reminder that these ideas are by no means universal to those attached to the classical liturgy -- and not even within the SSPX. Further, pushing aside those extremes, there is indeed an aspect of critique. But ultimately the root of that critique is not so much of the Second Vatican Council, as it is the interpretation and implementation that some have given it these past 40 years. While some may not agree with this critique, this is different than a rejection of it.
In this equation a few sobering facts must be remembered. While there are traditionalist extremes which need to be expunged, likewise as well as there extremes related to those whom have embraced a certain sort of modernity, complete with its temptation to syncretism, relativism and secularism. This critique is not the parlance simply of a traditionalist crowd, but is a part of the assessment and critique of the likes of the Holy Father, both present and previous. This aspect must be remembered in the consideration of this question. It is, in fact, what has effectively caused this situation.
As well, there is a liturgical issue. There has been a problem with the way in which the liturgy is oft-celebrated today. But more than that, as Ratzinger identified, there was also a serious issue with the handling of the post-conciliar liturgical reform, one which has contributed to a mindset still in need of correction and which has abused the sacred liturgy. This as well has caused this situation.
In the question of the Motu Proprio, there must be a full and real accounting of the various factors that have led us to this point.
With regard to the extreme views on the fringe of the traditionalist movement, let us also recall, that these aren't likely to be tempted by this Motu Proprio. It is precisely because, for that subset, the Mass isn't the only issue, that they are unlikely to come back into communion. -- so long as they hold to those ideas. As such, is this any real reason to oppose the Motu Proprio? Is this a real threat? In fact, to date we've heard two contradictory reasonings against this Motu Proprio. One is that it will bring an undesireable element back into full communion. The other is that it precisely won't bring them back.
So then, to what are we left? We are left with earnest Catholics whom are concerned with the interpretation and implementation some have given to the Conciliar documents, and we are left with Catholics whom are dismayed at the post-conciliar liturgical reform which went beyond its mandate and placed a rupture within our liturgical tradition, while at the same time relegating that tradition to an effective banishment. But these ideas are shared by our Holy Father, so they are hardly extremes.
An honest counter critique of this opposition to the Motu Proprio would suggest that indeed there is an ideological battle at stake. But that battle seems to me more primarily not about the extreme fringe of the SSPX, for whom all must realistically ascertain will not deem sufficient this Motu Proprio, but rather about a defense of the post-conciliar status quo as we now have it. In short, a defense of the implementation as it actually happened and of the interpretation that many have given it. Or in other words, a defense of the "spirit of the Council" that has been so oft quoted.
I suggest this due to the critical comments with regard to the reform of the reform which we have heard, or in relation to critical comments about the liturgies of another age. Such comments are demonstrative, it would seem, of a hermeneutic of rupture. A hermeneutic which is not understood as being rupture necessarily, or at least not undesireable rupture, and which seeks to be defended.
This isn't to negate the fact that there may be various legitimate concerns to the question, but I am not certain those concerns aren't being used to cover up other ideological matters which don't strike at the heart of the traditionalist fringe, but rather strike at the heart of those within the mainstream who seek a proper and balanced implementation of the Second Vatican Council, and who wish to restore a hermeneutic of continuity in our local parishes and Catholic institutions.