What I can tell you is the game plays much faster and cleaner. Reserves make a massive impact on the game, as does terrain, so defensively there are a lot more tools at your disposal.
Because of reserves and certain other elements, T1 flows much faster, as in all my games at least, everyone took as full advantage of reserves as possible. Because it comes at a CP cost its not an easy choice, but very important. The way reserves works is very different than previous editions. Its hard to describe without giving away too much, but its very dynamic and markedly different than previous iterations of reserves.
The game in general is much, much faster. There are a couple of habits you need to develop to keep it fast, but myself and playtesting partners barely had to crack the rulebook after the first 2 turns. So the game is fast, and the familiarity with 8th is still very much there.
The terrain rules are super important and will really be a hobbyists delight. If you've got a guy in your group that loves making terrain. You will be happy.
I found my unit selection for 9th has been very much different, but for the better. I felt 8th rewarded the long time collector of single faction, way more so in 9th. I feel really rewarded for sprucing up all my old favorite models/units to my current painting standard.
At times I've felt really jaded about having to reconfigure my collection, but if you've had a somewhat measured approach to collecting and have your core really solid, you'll hit the ground running.
On the purely subjective side, there are a lot more stratagems and special abilities that really get the blood going thematically. In playtesting there were a lot of moments, where I was just like "YES! This is what my army is supposed to feel like/do!" Its hard to describe and I'll be able to get into on my podcast eventually, but there are some really awesome fist pumping stratagems that just scream rule of cool and are very good but not game winning on the table.
Finally Crusade is just incredible. All the competitive guys in my team have dropped tourney play for Crusade because its so damn addictive. Its fun, easy to get going and all you need is a very basic story for the campaign to just sort of take off. The way armies interact with Crusade will very much help craft the story for you. When more is revealed I'll point to exactly what I'm referring too, but if you've longed for a great campaign system, to have a high stakes and meaningful series of games Crusade is your jam. There is a weird cross-over with tournament play in that, you feel like you are playing for that big win in every game and the time between sessions is almost as fun as the games themselves.
I know a lot of that is vague but I do have a very serious NDA on what I can say. And for those of you guys who've chatted with me throughout the years, you know all this comes from the heart. If I didnt think 9th was good, I'd be silent. There are things that still rankle me, but on the whole Im in love it, its my fave edition thus far.