Huh. My initial impressions for some of the more interesting changes are that:
Phantom Lancer appears somewhat nerfed. Well, okay, it's a rework, but he no longer has the best agility growth in the game, and no longer has an easy escape mechanism, but his clones have now become much more manageable, and he's not quite the "farm 40 minutes and end game" terror he used to be.
Riki got a bit of a rework (permanent invisibility from level 1, and health regeneration from that? Guess I don't have to buy Ring of Health anymore...). Not sure if this is a nerf or a buff yet, though I'd almost prefer the 5 second blink strike to the maximum 5 charge one as an ultimate.
Treant got one of his old abilities back with a twist on Aghanims. I sorta missed his ability to spam full vision on a jungle from early levels.
Lone Druid's bear now has mana, so you don't have to spend an additional 1000 gold to get an energy booster, and either grab Midas or troll people with Dagon, which can be used outside of normal attack distance (where it would usually prevent him from attacking). I'm wondering how Necronomicon (Bearonomicon? Necronomibear?) can be utilized on him. I see a possible nerf coming to that soon.
Chen can mindcontrol Ancients with an Aghanims. Never thought I'd see the day that'd happen. Better for charging a base than ganking, with how beefy they could end up. Too bad he can't grab Roshan. But a couple Black Dragon ancients will clear out a wave with their splash attacks in no time at all. Maybe Chen will actually get played now.
Clinkz' Searing Arrows no longer counting as an attack modifier. Lifesteal, Skadi, and additional damage anyone? In any case, it opens up options like Desolator, which weren't available to him before.
Bloodseeker now doesn't seem to be quite the pubstomper he was previously, but now is even more reliant on his ultimate to be able to land his new silence, but gets Thirst bonuses regardless of whether an enemy is visible to him or not, unless I'm reading the notes incorrectly. Bloodrage affecting spell damage is a nice opportunity for a Lion or Lina to instantly gib an enemy if used well via teamwork. No passive life regain on kill makes me a sad panda, as he doesn't get an easy mid anymore.
I'm interested to see how the damage type rework will play out as well. It definitely seems to simplify some things that had gotten a bit messy over the years it was developed in Warcraft 3's engine, then ported just for the sake of keeping it the same. I wonder how many dirty tweaks they ended up working out after the removal of some of those abilities. They also seem to be easing out of the comfort zone of 522 maximum movespeed, as in order to get the Warcraft 3 engine to work with it, some hacking with JASS needed to be done.
In any case, I'm glad to see that they're starting to move away from the limitations of the engine that the game was based in for far too long.