NMCS Texture Pack
There's a morbid sort of beauty in a decaying landscape, and the NMCS Texture Pack makes New Vegas even more bleakly attractive. Everything from roads, trees, buildings, vehicles, and other objects have been retextured (sky, water, characters, and weapons are untouched). More detail usually means a performance hit, but there are three different levels of quality to choose from if you have issues running the biggest textures.
Your gun is broken, your knife is dull, and your armor is in tatters. How are you going to fix them? Traditionally, by finding identical versions of the broken items and cannibalizing them (or by paying a vendor to fix them for you). With Alternative Repairing, however, you can break down other existing items into base components, then combine those components into replacement parts for your gear. It's a nice balance of giving you more repair options while requiring some extra effort, and most of all, it makes you feel like a real DIY enthusiast.
If you find New Vegas a little too easy, this mod by Joshua Sawyer (the actual director of the game) is here to help, and that help is gonna hurt. This massive series of tweaks means you'll gain less experience from combat, you'll level far more slowly, you'll be able to carry less, and you'll be far more vulnerable to damage. Eating, drinking, and resting are now a top priority, and ammo and stimpacks—formerly lighter than air—will now weigh you down. There are dozens of changes for a more challenging experience, making FNV a true struggle for survival, just as it was originally envisioned.
Note: This mod requires every last scrap of official DLC to run. The download is on his page in the "Links" section.
NV Interiors Project
Sure, you'd expect to find a bunch of boarded-up buildings around the wasteland, but seeing as how you're a super-powered explorer with an arsenal of explosives, you'd expect to be able to bust your way inside, too. These two mods, one for Wasteland buildings and one for Urban structures let you inside those formerly impenetrable buildings so you can explore, loot, and maybe even find a few secrets.
New Vegas has its share of nasty creatures, but after a few romps across the map, you'll probably grow bored of fighting the same monsters over and over. This Monster Mod adds a monstrous number of new beasties: some are tougher variations of existing monsters like dogs, scorpions, and ghouls, but many are entirely new, such as giant two-headed axe-wielding zombie mutants. With over a hundred new monsters included, every trip through the desert will provide you with a fresh and terrible surprise.