I'm kinda bummed out; I keep hearing conflicting stories about the performance of 1.8 and 1.9 vs 1.7.
I have no experience with 1.9 yet (just started porting to it) but for me 1.8 performance is way better then 1.7 performance (even with CoFHTweaks or FastCraft on 1.7)
MC 1.8 and 1.9 add performance increases in some areas, but performance decreases in other areas. In particular, stutters due to GC / etc (and even the pauseless GCs have performance decreases while they run - and G1 takes long enough to pause to do new gen collections to often cause a stutter!) matter when they add enough overhead to cause you to miss a frame deadline. If you have enough headroom that that doesn't happen, MC 1.9 is generally better. If you don't, have "fun".
Unfortunately, what it means in practice is that, generally speaking, lower-end computers get worse, and higher-end computers get better.
Well I think the nail in the coffin for 1.8 just arrived. There is a download for forge 1.9 now.
Let's see. In addition to most of what I said about 1.8:
More options for mapmakers. Somewhere between irrelevant and potentially-detrimental for me (should servers decide to use them, for instance.)
More tweaks which are interesting, but not exactly essential (subtitles, world selection screen, etc).
Combat mechanics. Oh dear, combat mechanics. We've gone from beta, where you bashed at each other and juggled inventory until you ran out of health regens due to unstackable items to... 1.9 where you bash at each other and juggle inventory until you run out of health regens due to unstackable items.
And given the choice, I prefer beta's implementation. Or rather, what I wish it was would be as follows: you can swing as fast as you like, but each swing damages your sword slightly (Just enough so that spam clicking rapidly degrades a sword) (a hit degrades it more than a miss), and there is a short invincibility that affects damage only (so you can still knock players back), and only acts as a reduction (so if you get hit for a 1/2 a heart and then 10 hearts you still take 10 hearts of damage - 1/2 from the first hit and 10 - 1/2 = 9 1/2 hearts from the second). Ideally, I would want a short slight reduction to range after a swing as well. Not much, just enough to prevent "knockback auras" from being the be-all and end-all.
Dual-wielding isn't entirely thought through, although it's a relatively minor matter.
Shields are annoying, especially when watching video. Takes up too much of the screen. And people will complain about resource packs to fix it being hacks.
Treasure enchantments... I can't figure out why you'd do that. If it's to force players to do other things, it doesn't work well due to the inclusion of fishing.
Mending... Why would you have mending and yet still block repairs on anvils?
The end stuff... I feel neutral about. I liked the end being an island alone in a void. If you're going to introduce multiple dragon fights, do so by adding new islands a million blocks away or something. Like the inverse of the nether - one block in the overworld is 1000 in the end or something.
I enjoy the water splash potion, although using gunpowder to craft something to put out fires seems silly. (Although I am aware that it is actually semi-realistic)
The arrow stuff... why would you gate that behind lingering potions? Especially for servers...
I really dislike the alternate block toggle being removed.
The elytra... among other things the crawling stuff is silly.
Health regen just brings us further toward the bad parts of beta combat.
Mob spawning changes are a plus.
The nether portal invulnerability is a hack.
The brewing stand change feels like a demand to return to the nether. I would prefer a request. Again: inconsistency, considering that nether wart used to only grow in the nether but now can grow in the overworld.
I enjoy being able to fill buckets from cauldrons. Especially useful for skyblock / puzzle maps.
I don't like the new sounds, generally speaking, although I am well aware that is because I am used to the old sounds. As this can be changed, it's not too much of an issue - although it has made watching videos somewhat painful.
Cobwebs... I liked the silk touch-only thing. Although I can understand why they changed it.
I enjoy the tripwire change.
I dislike the cooldown on ender pearls. Makes them much less useful for panicmode "I've fallen and will die" situations, while not actually fixing the problem it was intended to solve.
I don't like the heart particles on hitting mobs.
The boat changes... Better than before in most ways, but the "unable to ride upriver" bit is absurd. I don't mind too much not being able to ride up a waterfall. But having a boat sink to the bottom of the ocean because of an 1/8th slope is absurd. Especially given it's entirely made out of oak/birch/etc wood (which all float).
And that's not getting into the actual bugs.