I love the fact that the 3-D items will be added including fireworks and all of these enchantments that will make gameplay alot more interesting for some people, also this should mean many more interesting mods and maps to be created in the future.
3D Items = Unbearable lag for those of us with average computers.
Most of the lag is from horrid engine optimization (compare MC's speed with all the clones out there). Mojang was working on fixing those issues, but not sure what happened to that project. I think bits of the optimizations have made their way in, but there's a lot of work to do yet.
The new Redstone engine and the new Lighting engine (that vanished) are both part of that optimization. There may be other optimizations coming that have allowed for the 3D floating drops too with minimal impact (pure speculation on my part)
Mojang have said they will add something better than optifine, so hopefully we will get much better custom menus and features. You may be able to have fat clouds and normal item rendering soon.
I sure hope so. I guess maybe I'm just a nostalgic with this sort of thing... Though so far, I don't think it's likely that Mojang will change much on the menus. They like to add things, not improve on them.