My two cents on 6.78
Note: I'm only doing ones I find to be "big" out of the Dota 2 heroes as I have not had enough experience with the heroes not in Dota 2 yet to know what these changes truly mean.
The new heroes: Can't wait to see these guys in practice. They both sound pretty interesting.
Ancient Apparition - Scepter is HUUUUUGE. Definitely can see him being put in a 3 position to farm out that Scepter. Comboed with something like a Venomancer, that's going to be extremely deadly.
Batrider - A necessary nerf, Firefly made it easy for him to grab pretty much anything with a Blink Dagger. That Flamebreak manacost increase might make people want to run Aracne Boots on him, but I'll have to see it in practice.
Bloodseeker - Giving him armor with Thirst is interesting, will definitely encourage diving with him. Bloodrage also being a purge is interesting, Ghost Scepter will be pretty useless against him now, since his damage was all right click before. On top of that, Pugna can't Decrepify him to shut him down. The Rupture damage penetrating BKB now, that is pretty big. I'm not sure if it will be enough to make him viable though. We will see.
Bounty Hunter - Thank god they conserved his mana a bit. After Track got the armor reduction taken away, it's good to see that we got a reduced mana cost to compensate for that.
Broodmother - Incapacatating Bite miss chanced is 60% at level four? Now spawning four spiderlings all doing this at level four? BM may become viable again.
Clockwerk - A needed change, that's for sure.
Crystal Maiden - Maybe people will consider more points in Aura sooner now. Those new mana regen numbers are pretty nice.
Dazzle - I can see new Shallow Grave being abused as hell with Lifestealer...
Death Prophet - The Silence AoE increase probably makes one early point in it a more viable option.
Doom - That Scepter upgrade is huuuuuuuge! Not only that, it disables a lot of passives that countered it. No longer can PL just sit there beating on him during Doom and be okay. Putting Doom farming with a Midas may be viable, but there were no armor changes. He'll still need to do something about that problem. However this may put him at least somewhat viable.
Dragon Knight - THANK YOU ICEFROG! Now people don't have to sacrifice not having their splash so they can push down towers. It did always seem kind of dumb to me that his level 2 upgrade on his ult wasn't really much of an upgrade.
Earthshaker - "Those are some nice illusions you got there PL, be a shame if something were to happen to them."
Enigma - Will definitely make ganks from the jungle much easier and more likely to be successful.
Timbersaw - Glad that Chakram manacost is rescaled. I always felt so useless as Timbersaw before Bloodstone. Glad to see he has more Int too.
Gyrocopter - Going to slow down his farm, but I see him still being picked quite often.
Huskar - Not sure how I feel about this one. True, his problem was often he would Life Break in, and then some support would pick him off with a nuke when he has no health. It's likely now a support's damage probably won't kill him either. Taking away the extra damage though, that may have gone one step forward and two steps backward.
Lifestealer - This will make him much less effective early game (as most don't max Open Wounds first or second) so he can't slow from forever distance away. I imagine people will still take one point in it just in case they need the extra lifesteal off an Ursa camp or something.
Lion - TWENTY SECOND COOLDOWN ON AGHS FINGER! That's going to be ridiculous. Also glad to see that Mana Drain is a little more useful. If someone got drained they would just walk away and have most of their mana intact before.
Lone Druid - Saw this coming. Also, I thought Armlet Bear was a bug only in Dota 2, and did not work in Dota 2. Was I incorrect about this? 300 bounty on the bear makes that pretty much a hero kill. I always thought 100 bounty was too low, since Visage's familiars were also 100.
Magnus - Kind of a "slap on the wrist" nerf, which is good, as he didn't really need anything major. All this means is he isn't going to kill two or three full health heroes with a good RP early game. He'll still get picked because of all the amazing things you can do with RP.
Medusa - Sure glad she's going to be removed from CM, because she's going to be the go-to carry with that Stone Gaze rework. I'm just imagining this with something like a Ravage. Hell people may run Shadow Blade on her now to get her in position, Tide Blinks in for Ravage, Stone Gaze, everyone is stone form for her to split shot down. Also 50% damage bonus is absurdly overpowered, I can tell you that already. This is on the hero that's going to be splitshotting down the entire team.
Meepo - I can't remember who said it, but someone in here suggested the other day that Meepo should get more Magic Resistance and he's good to do. Well you got your wish! I hope Meepo gets picked more often. Even before this buff he was still awesome and an underrated hero.
Mirana - Much needed buffs. Glad to see there be more of a payoff for making the long distance arrows, one of the most difficult skillshots in the game. The Moonlight Shadow Buffs are appreciated too, as well as to Starstorm.
Necrolyte - King of the killsteals! Seriously though, that new Aghs buff is ridiculous! Definitely see this becoming viable, even if he's run as a 3 just to farm his Aghs for that.
Nyx Assassin - So I imagine this will change people's builds to max Mana Flare before Spiked Carapace now.
Ominknight - Game. Changer. That's to be absolutely ridiculous paired with something like a Treant Protector.
Brewmaster - Crits from the Fire Panda, oh man! Not only that, right clicking the things will be a gamble now too.
Pudge - FRESH MEAT! This is pretty huge. He'll spiral out of control reaching the "unkilliable" status much faster.
Pugna - 50% damage from Nether Blast on towers is a pretty big deal! Also people are really overlooking the possibility of Decrepifying the Tombstone. Remember when your team is yelling at you "Attack the Tomb!" and then you can't? Yeah, it's still spawning zombies while you can't do a damn thing about it.
Shadow Fiend - 50% damage reduction on Requiem! Wow, this is going to help offset the problem of SF's low health for sure!
Silencer - That Scepter ult sounds amazing. With a seven second silence and free Curse, I can see this becoming viable. Does anyone know if the Curse from the ult will stack with the actual Curse spell though?
Skeleton King - No longer the one-button hero, the temporary health steal will keep things very interesting and give him an edge, especially when he's down to fisticuffs against someone like Lifestaler. The changes to Reincarnation are welcome too.
Elder Titan - Why did they nerf this hero that nobody used? That seems kind of stupid to me, but maybe I'm missing something here. The Natural Order increase is nice, but that loss of flying vision really sucks.
Troll Warlord - Both of these changes are definitely for the better.
Ursa - That's huge, and going to make him much more effective in teamfights. He will not be completely screwed if he gets disabled at all to stop him from attacking one target.
Items
BKB - They have really nerfed BKB a lot the last few patches. A four second BKB now. I can see rebuying (since you can't sell your old one) BKB being a lot more common.
Bloodstone - This is really going to piss people off when they have a way to instantly deny themselves.
Bottle - I was of the camp that bottle crowing shouldn't be completely removed, but nerfed slightly. I think this is definitely the right decision.
Dust of Appearance - Really going to help against that pesky PL, that's for sure! It will also help against Shadow Blades too.
Gem of True Sight - Everyone will be scrambling for the gem that gets dropped in a fight now. Probably a necessary change. I know it's going to help me when I'm not sure what to do with a gem/don't have space for it. Usually I destroy it by default, but I'll always make sure I can pick it up or someone else can. I hate having to quickly drag items out of inventory.
Observer Ward - Only one pack of wards at the start? That really sucks for whichever side lane doesn't get them. I really wish they would make stacked items disassemblable, such as TPs and Wards. At least top would be able to take one ward and bot could take the other that way.
Quelling Blade - This solves your uphill miss problem on dewarding now. It also lets melee heroes that have one help in the dewarding process.
Other changes
Drafting should become a lot more interesting now.
4-man and 5-man Doto is now much more encouraged!
Six minute buyback cooldown, yikes!
I like the change in tower bounties, good for everyone, especially the supports that won't have much money.
That change to catapults, THANK YOU ICEFROG! Now maybe I can actually reliably last hit those things.
Swapping a randomed hero costs you 100 gold, does this mean you start with 703 or 503? It's a bit unclear...