Yes, gameplay is near identical to Fallout 3. Items will exist physically in the world, etc. etc. just like in Fallout 3.
The only changes are this:
-New Story
-New World to Explore
-SPECIAL and stats have more effect on the game
-There is a new skill called Survival, and Big Guns and Small Guns have been combined into one skill
-Companions have more story and personality, and each grants a unique perk and has there own quests, and are more easily controlled through a pull-up menu - you no longer have to go into dialogue with them to tell them what to do
-There is an new optional hardcoe Mode that you can select that makes ammo have weight and forces you to eat/drink and sleep or suffer penalties
-Faction uniforms function as disguises
-Less of the game world is leveled - mainly only on the main storylines paths
-Level Cap starts at 30 out of the box
-Perks are now acquired every 2 levels instead of every level
-Magazines and skill books only give temporary bonuses to stats
-There are no bobbleheads that raise stats this time, only snowglobes, and those don't give stat bonuses, only an in-game reward once all are found
-Damage Threshold is now used on Armor instead of Damage Rating - in other words, instead of armor blocking a certain percentage of damage, it will block a specific amount of damage
-There are now different ammo types for each weapon, and you can make your own ammo using workbenches
-There are new status effects in the game such as poisoned - like from a Radscorpion
-You can make items or cook food using the Survival skill and campfires
-The game has a definitive ending like Fallout 3 had before the Broken Steel DLC, but the ending will be much longer - detailing exactly how your actions shaped the world and what happened because of you in each community you visited in the game
-Low Intelligence now affects your dialogue responses like in the original Fallout games
-Weapons have Strength requirements to use them without a penalty
-NPCs no longer react to you looking at locked objects or kicking physics objects - no more "Hey! Watch it!"
That's all I can think of at the moment, I probably forgot something, but nothing for you to get worried over. To the contrary, many things to get excited over!
EDIT:
I don't think they'd mess with your ability to leave items laying around in the world. What did worry me on a similar note was a comment from one of the developers about how they didn't like all the clutter in fallout 3, random tin cans and the like laying around, and how they'd cut back on it in New Vegas.
But from what I heard from Pax there's still plenty of junk around for hoarders like me, so I'm not so worried anymore. If he just meant getting rid of things like cans in the middle of the desert that literally serve no purpose, I guess I won't miss them that much.
There
was to much junk and clutter laying around. Just because people are poor doesn't mean they wouldn't keep a clean house, however much of a run down shack that may be. Sure, they may save tin cans for other uses, but they would do so stacked on a shelf, or all in a box. If a house is a mess with trash strewn everywhere, that should be because the NPC is a slob or the house abandoned to the elements. Just because bombs dropped 200 years ago doesn't mean these people can't break out a broom! Moira's shop is awfully filthy for someone who spends all her time sweeping.