Keep in mind that the game isn't released yet, so this is pretty much based on prerelease material, which obviously doesn't tell the whole story.
1. Leveling (will it be like in Fallout 3 or Oblivion)
In a way, neither. Skill points are a lot less plentiful than in FO3, which means that even at level 30, one character may be much different than another, unlike FO3 where by level 30 every character basically had most skills maxed out.
It's closer to FO3 than Oblivion though.
It should be pretty much the same, with the caveat that the NV GECK includes NV features, like DT (also has DR as an available field...), as well as some improvements like a better dialog editor and higher dialog character limit.
Should be NVSE compatible when NVSE is released, assuming it doesn't include all the FOSE script functions natively.
Massively improved across the board. Now to command them you have a simple and intuitive wheel instead of having to wade through menus. Also, each companion gives you a perk when they're following you (eg Raul slows weapon degradation, Eyebot lets you see stealth/cloaked enemies more easily, etc), and they've also got their own stories, characters, and quests.
It's doubtlessly gotten a lot complex. In terms of gameplay we have the new survival mechanics in hardcoe mode, several new ammo types, a large number of new guns, the ability to modify weapons with scopes/silencers/extended mags/other mods, plus the fact that companions are now more than just a walking storage bin and gun platform.
In terms of story it's also a lot more complex. We've got the conflict between Caesar's Legion and the NCR, with Mr. House lurking in the shadows. We've got sideplots galore, and they've promised us real Fallout 1/2 style endings with consequences, making each decision you make matter, unlike the garbage in FO3.
Eh... Nobody knows. I'm inclined to believe it's less casual than FO3, but FO3 was so casual it was stupid, so this is an improvement to me.