That's a very food primer. I have to admit I'm surpirsed players will read a 50 page primer, though.
I try to make a strong opening. Don't start with "this document does this and that", but rather go straight to the strongest and most appealing hook. Hook these new players in from the first line. So I'd begin with something like "In Ars Magica you play wizards in a fantastic version of historical Europe, bringing to life all the tropes of fantasy wizards while enjoying the depths and familiary that historical Europe and authentic myth and folklore brings". And then go to "Some of Ars Magica's strengths" and so on.
I'll also be mindful of the order of presentation. You talk about magi, the Order of Hermes, and so on before you explain these terms.
When describing the Houses, I like to emphasize more stereotypical magi. This really works only for a specific saga - if it narrows down a House to them, for the most part, then you can describe the House as such. Your descriptions are more general, which I think would actually work better for a general primer. To give you an idea what I mean, perhaps something more like:
House Bonisagus is made up of researchers that seek to advance Hermetic magic. Many are primarily engaged in laboratory work, striving to invent new spells, enchant new magic items, and so on. Others seek out legendary ancient or exotic magic, in the hope of integrating it into Hermetic practices and thus extending the Order's magic. A few Bonisagus magi are concerned more about the Order than its magic, seeking to protect and maintain the Order and its secrets.
House Bjornaer is a mystery cult that follows great Magical animal spirits. Bjornaer magi include shamans that follow the animal spirits in vision-quests into the Magical places and even the Magic Realm itself, mystics that seek to awaken their inner, Magical, form, and protectors of the forests and all wild places and of Magic itself.
Is the telling of the Order's history in such details really needed? Won't it be better to give a more abbreviated one? Is this really something every beginning player needs to know to start playing?
I also like to give more feeling to where are the different auras in the world, and how they feel like, especially the prevalance and nature of supernatural entities, spirits, realm-aligned folks, and hedge traditions. e.g.:
Magic auras still cover a lot of the land – these are the primordial forests, glades, mountains, deserts, oceans, and lakes, and a myriad of smaller auras and regios. They are associated with extraordinarily large beasts (Animals of Virtue), animated trees, magical herbs, chimeras, elementals, giants, and dragons.
...
Magical traditions are most common in the rural outer reaches of the Dominion, in the small villages abetting Magical lands. These often include people with a single Supernatural virtue or flaw, such as Font of Knowledge, Ghostly Warder, Lycanthrope, Magic Sensitivity, Premonitions, Second Sight, Skinchanger, The Gift, or the Visions flaw. These are known as "Magic affiliated" or "Magic touched" characters. Note that many people also have virtues related to other Realms of Power, such as Enchanted Ability (Faerie) or Guardian Angel (Divine).