Well, let's see...
The advencement and character creation rules are more in alignment under 5th edition than any previous edition. There are actually handy and workable rules for creating older characters without having to go for "Well, I guess that looks right".
The combat system changed ... again. That could be good, bad, or indifferent, depending on how you look at it. If find this particular change just a change, not really either a step forwards or backwards.
Many people like the new breakdown of Virtues and Flaws into Major and Minor, rather than point cost. Some do not. This is a matter of taste. I find this new method is easier for new players to grasp, if that is worth something to you.
There are many spells that have moved about, nothing new there. Again, this happens every edition.
The guidelines for creating new spells are clearer even than those that appeared in 4th edition. I really like this. Devising new spells and device effects is pretty easy now, although there will always be some arguments on the fringes.
In play, I find that magi are not so much worried about dying of old age, but rather of being "warped". The Warping and Twilight rules are good, consistent, but also utterly unforgiving. It is quite possible for a character in a relatively short period of time, if he is not careful, to gain extra Flaws simply due to magical overload.
Admittedly if you have all the 4th edition material you will find things missing in the new edition -- there are not as many books. One large hole that was filled had to do with creatures. The pdf on mundane creatures available in AM section of the Atlas site is a "must have"; thankfully it is also free, so no problems there. However without that pdf, I think most new players would have been lost.
In general I really like the new rules. I am also quite happy with 4th edition in most areas. Given a little work-up, I could easily play 2nd edition again; I'd skip 3rd for the Realm of Reason and the Passions if nothing else, but I could even play in that. In other words, I am an AM junky...