What changes have you noticed in the Definitive Edition rules (that weren't already in the errata at Atlas Games | Ars Magica Fifth Edition Errata)?
It looks as though Container targets are a formal thing now, and Fast Caster finally helps fast casting!
Take a look at AotH.
Mostly these aren't changes so much as clarifications. But, yes, containers are now formally there, which is helpful. Also, look at Circle in the container section, and you'll see it's clear these are not supposed to be infinitely tall cylinders.
Ah, you spotted that one. I thought people would find that much more reasonable.
Manuscript apparent contradiction question
reading the manuscript 07-Hermetic Magic i find under the Muto heading "Muto magic cannot affect the properties that something has naturally, although it can add other properties to them to mask their effects." Under Perdo I read "Perdo destroying the ability to burn anything, and Muto granting the ability to burn only wood, a property that fire cannot naturally have" This seems like a contradiction. Am I missing something?
I'm not seeing the contradiction? Muto magic cannot alter the natural properties of a target, only add properties, potentially suppressing said natural properties, but the property remains for when the spell is over. Perdo could remove the property of burning from a fire, whereupon Muto could add the ability to burn wood, thus resulting in a flame that can only burn wood, but wouldn't catch, for example, your hair or clothing on fire (barring Twilight Scars or a Herbam specialist with an odd fashion sense).
A lot of people misinterpret this. Wool can burn. This Muto isn't to change the wool, but to change the fire so that it is hotter so it can burn stuff it normally couldn't, while Perdo is removing its ability to burn all the other stuff. You can't do it just with Perdo because you need to change the fire to be able to burn the wool.
I was sure I commented about the confusion this example brings about, hoping it might change. Also, this isn't a new example. It was in the original book, which is where I'd commented on it.
I don't see where Art of Memory lists the number of foci you get?
Is that a rule change or an errata candidate?
I'm glad to see this finally explicit!
Rounding
The rules for Ars Magica sometimes involve division. In most cases, a rule specifies whether you should round up or down, but if it does not, round down.
It wasn't until I saw this comment that I realised I had hallucinated this rule already being in the core book! Had to go back and check.
It could be a bit clearer. Is it meant that you round down all fractions (1,8 is rounded down to 1)? That's how I read it, but some may interpret it differently (that only 1.5 or below is rounded down to 1).
An example might be good for that.
It couldn't be clearer.
I don't see any way any reasonable reader could interpret it in a way that doesn't apply to all fractions.
If you round down, you round down. That very clearly and obviously includes results like 1.8 as well as 1.4.
It's clear to me. But I've met people who would argue over that. 
I'm with ErikT on this, and reminded of "What the Tortoise said to Achilles", Lewis Carroll's masterpiece.
It has changed. It has changed to "wood", which does burn normally, and includes an explicit note that Muto is adding a property that fire does not have naturally (that of burning only wood). I am not sure how this is supposed to be a contradiction. Muto says that Muto cannot affect natural properties, and Perdo says that it is affecting a property that is not natural.
That's an error. I simplified away from Memory Palaces, and since that limit was only mentioned in that section, it got dropped.
Oh, yay. I totally missed the "d" v. "l" change in there. It looked so similar to what I'd read so many times I totally misread it. Thanks for the change!
I think people who would argue over the current phrasing are not people I want to play Ars Magica with. There are far more open-to-interpretation rules than that one, and at some point I want to stop arguing the toss and play the dang game
Just in case I'm not the only one confused:
i was totally lost, since I had nothing, so I connected to backerkit and saw something about an email confirmation. I checked my mail ( I seldom do), and here it was: an email titled "early bird rewards" 
How's the old question of if Wound Penalties should apply to the Soak roll you make when someone casts a damaging spell on you? We've been arguing over it on the Discord but I'd like to see if there's a definitive (pun intended) stance on it.
There is - the Obstacles chapter specifies:
Characters may have any number of any type of wound, in any combination. The character suffers a penalty to all actions (rolls and totals) equal to the sum of all penalties due to his wounds, and the activities he can safely undertake are restricted (see Activities While Injured, below). Note that Soak is not an action, and thus does not take the Wound Penalty.
It looks as though more Flaws have both minor and major versions now:
"Beloved Rival" seems to have a Major Story Flaw version as well as the original minor flaw version. Oddly, it does suggest that a character might have both the Major and the Minor version simultaneously as two separate flaws:
If the character takes on the role of the beloved rival for another character, then this is a Major Flaw. The character generates stories by causing trouble for their rival, and also has to react when someone else tries to harm them.
Such relationships are a tradition of House Tytalus, and may be mutual. When the rivalry is mutual, the troupe may allow the character to take both the Minor and Major versions of this Flaw. This will mean that the rivalry will be a very important feature of the whole saga, and thus the whole troupe must consider whether they want that.
I'd personally be more inclined to go for the Major version and the Protection virtue, assuming the rival was sufficiently powerful to qualify for it.
Compulsion, Oversensitive, Reckless and Vow were previously minor Personality flaws, and can now be either minor or major. Pagan has also switched from being a Major Story Flaw to a minor or major Personality flaw.
On a different note, where was The Evil Eye flaw previously? There were a number of references to the concept in The Sundered Eagle, and it was one of the options for a type of Folk Magic, but I don't remember a specific flaw by that name.
Incompatible Arts no longer excludes Corpus and Vim from being in the excluded Art combos.
Compulsion, Reckless, and Vow had already been changed to Minor or Major through errata.
The Evil Eye is from RoP:D p133 and p135
We have rounding rules now? That's a tragedy. I think it makes the game worse not better. I can't think of a number in our game that benefits from the extra step of rounding. We have an additional rule so we can go through additional steps to get a final result that's more vague than it was before.