Nothing. At least, nothing worthy of the complete reforms of the liturgy as happened in 1970. The 'problems' of it were:
a) It expressed Catholic theology in it's entirety without obfuscation and apology
b) It was ancient, going back in its essence to St Gregory the Great
c) It had a complete and beautifully Catholic Eucharistic theology
d) It was uncompromisingly Catholic in it's wording, rubrics and symbolism
All these were problems for a lot of the liberal, Freemasonic-inspired, hippy Marxist Bishops of the 60's who were caught up in a mania of ecumenism, univeralism and modernism. For a complete history I can give you some books to read which are enlightening for any Catholic at just how unbelievable the changes were not only in their very form but also the reasons behind them (all thoroughly anti-Catholc). Cardinal Ratzinger befrore he became Pope wrote about it a lot too. Michael Davies was inspired in his writings and research. Many priests today are getting wise to the complete destruction of the Roman Rite made by the liberals in the 60's. If you want a simple kind of history and comparison
this (lengthy) article does a very good job. As mentioned, this is a huge topic and full of controversy right now. I am not SSPX; but I am one of those formerly-sidelined and hated Catholics who thinks the Traditional Mass is far superior to the new Mass. Not that I think the New Mass is invalid or cannot create good and holy Catholics.
If you'd like more articles I can whip up tons!
Here is a website with a lot of excellent resources on this issue (specifically under the headline: liturgy).
The thing is that the Mass was not perfect. We can never have a perfect missal in this life. But that is why we allow organic, traditional development that takes years. We don't just form a 'liturgical committee' composed of self-confessed liberals and create a new Mass (which is what happened). The Tridentine Mass (a bit of a misnomer since it has been properly and organically revised many times since the 16th century) does in fact stand in need of a few good changes if done correctly and with due order and process. The readings can be improved; and I am not averse to them being in the vernacular. Some other things can be restored (Holy Week for example) and other things changed which are not exactly traditional to the Roman Rite. There is a continuing series of scholarly papers on this very topic right now available
here.
To please Protestants is probably the greatest reason; but others include a misguided 60's-inspired attempt at being 'relevant' (an error in itself, as the liturgy is always relevant precisely because of it's heavenly and supernatural character: it is relevant by being culturally irrelevant) and some of the fall-out from the liturgical movement that started in Continental Europe in the early 20th century. A lot of it had to do with post-WWII ideologies and ideas. A lot of it was misguided with good intentions but poor delivery. And some of it was diabolical in it's attempted denial and betrayal of Catholic liturgical theology and thought.
Again, the Novus Ordo when done correctly is not invalid or 'evil' as the SSPX would say. It is simply a deficient Rite when compared to the old Mass. If it is done properly - ad orientem, in Latin, correct rubrics and with reverence and traditional piety it can be okay though still not as good as the old Mass. However, there are probably only a handful of places in the world where this is done. The rest of the time it is psuedo-Protestant ad-libbed clappy-handhold patronizing stuff. It is a banal, on the spot product of it's time (to paraphrase Cardinal Ratzinger) and it has been the vehicle for liturgical abuse and sacrilege not seen since the Deformation in the 16th century. Communion in the hand, guitar mass, clapping, irreverence everywhere, lay people all in the sanctuary, themed masses, all this kind of rubbish is essentially anti-Catholic. And only in the last 10 years is this being recognized. Unfortunately this is a very polarizing issue because people take it very personally when they are suddenly asked to believe that Communion in the hand is irreverent or that the Mass is not about having fun and happy feelings but about serious worship and supernatural joy. Understandably it becomes painful and people see it is offensive to cite traditional Catholic liturgical practise and piety. But they're only experiencing what the faithful experienced 40 years ago when the old Mass was ripped away from them and within a month they were throwing away their Catholic worship in favour of Catholicism-lite where the guitars are suddenly great and the Eucharist is treated like a cookie. Which is why so many people became disillusioned. Terribly sad.
Anyway, a big topic, this. Really big.