Well it has the same graphics, the same controls, the same map system and the same HUD and menu's.
But apart from that it isn't like New Vegas at all.
Fallout 3 is set 200 years after the nuclear fire and it looks and feels like it's set 10 years after.
To clarify that, everything is destroyed and ruined and no one has done anything worth a damn to rebuild the place.
It has a very stereotypical cliché post apocalyptic feel to it, whereas New Vegas has a post-post-apocalyptic feel to it.
It has less quests, NPC's and civilized locations.
Fallout 3 is built around exploration and dungeon crawling, and while it's not what I want from a Fallout game it does those things well.
But heavy NPC dialogue? Many quests with moral choices to them? Don't expect any of that from Fallout 3.
It also lacks iron sights and companion wheel, it don't use Speech as often, it does not have STR Requirements on weapons, it's unbalanced and you will easily max out all skills and get overly powerful perks, it doesn't have the same crafting system (it's a lot more limited), it doesn't have more than 1 ammo type for each weapon (so no armor piercing rounds or max charge energy cells), it doesn't have a faction reputation system, it doesn't utilize armors as faction disguises, it uses a DR system over a DT system, it forces perks on you even if you don't want them, it's companions have a few lines then they basically stop talking.
Fallout 3 is not like New Vegas at all despite the seemingly similarities.
New Vegas is about a story and developing your character in a balanced way of role-playing.
Fallout 3 is about dungeon crawling, sight-seeing the world and gaining maximum Super Saiyan powers.
Fallout 3 isn't by any means a bad game, I had fun playing it. But it's apart from some of the very core things they're nothing alike.
So if you're going into Fallout 3 expecting something like New Vegas but better then you're going to be disappointed.
If you go in with a different mindset on the other hand you could very well enjoy it's wonderfully crafted world and tons of fun dungeons.
(Just don't try to make sense of the game-world though)
As to the other questions.
No it's not hard, just tedious with damage sponges.
Yes you start out as a kid, you actually start out as a baby.
And no, Speech isn't important in Fallout 3, it's quite the useless skill for most of the time.