UPDATE (NEXUS MODS):
In addition to the great survival / realism mods posted below, there are a number of mods that fit well with this collection, but are not yet available on Steam Workshop.

ASIS is a fantastic mod by the prolific and notorious Plutoman, which adds additional enemy spawns and improves upon the enemy A.I. tactics, for example, enemies will use potions and heal their allies with magic. NPCs will even be able to use spells from all your other installed mods!
Link: http://skyrim.nexusmods.com/mods/18436

Deadly Combat overhauls Skyrim's combat mechanics. You will no longer be able to stun lock enemies, and every attack has a chance to stagger. Most encounters will be much more challenging, as enemies react to the player in a more realistic way. When timed just right, wards be used to reflect shouts and spells right back at the caster.
Link: http://skyrim.nexusmods.com/mods/5485

Deadly Dragons is a mod that greatly improves dragon encounters by making them much tougher enemies, particularly at higher levels.
Link: http://skyrim.nexusmods.com/mods/3829

Occupy Skyrim adds epic additional random encounters to recreate the experience of a province that is truly at war, or with say a merchant under attack by a massive Falmer raid, etc. Greatly improves the game!
Link: http://skyrim.nexusmods.com/mods/12479

Real Wildlife adds many new variant creatures and enemies (and organs with new alchemical properties). Make sure to download only the "no food" version (file labeled "Real Wildlife - Skyrim 1_341 No Food") - so it won't conflict with IMCN.
Link: http://skyrim.nexusmods.com/mods/9778

Interesting NPCs is one of my favorites, a mod which adds 64 new voiced NPCs to flesh out the game world, with some interesting and fun quests.
Link: http://skyrim.nexusmods.com/mods/8429

UFO - Ultimate Follower Overhaul greatly expands upon the follower system, with features such as follower horses and spell training.
Link: http://skyrim.nexusmods.com/mods/14037

Unique Taverns expands upon the basic designs to make each tavern unique and memorable
Link: http://skyrim.nexusmods.com/mods/21553

Brehanin's Better Vampires greatly improves on the vampire gameplay and is highly customizable
Dawnguard Version: http://steamcommunity.com/sharedfiles/filedetails/?id=15998
Non-DG: http://steamcommunity.com/sharedfiles/filedetails/?id=87960882

A good fit with Better Vampires, Royal Bloodline expands the vampire lord perk tree in a lore-friendly way:
(Requires Dawnguard): http://steamcommunity.com/sharedfiles/filedetails/?id=89703769

DESCRIPTION:
This is a suite of survival and realism mods I've tried which all complement each other very well, and some fixes. You can click "subscribe to all" for this collection.

For anyone interested in trying a survival/realism playthrough, Chesko's Frostfall and Imp's More Complex Needs are must-have mods. IMCN incorporates a detailed and immersive needs system to track hunger, thirst, protein and nutrition stats for every food and drink item in the game.

IMCN is a perfect pairing with Frostfall, which incorporates a way to track your core body temperature as you traverse the harsh terrain and frozen wastes. You can customize the rate of exposure with Frostfall and there are a number of optional settings so you can dial up the hardcore.

The best feature of Frostfall is that it gives you a reason to seek out warm places, and provides a way to craft tents, bedrolls, firekits, cooking pots, all of which you can deploy in any location in Skyrim. LtMattMoo's Camping further enhances this aspect, by incorporating various types of craftable tents, tanning racks, chairs, a storage container, even a small mortar and pestle which allows you to concoct potions in the field (with a significant penalty compared to a normal alchemy station).

The Unofficial Skyrim Patch is another must-have mod, not only for those interested in a survival playthrough, but for everyone. This mod fixes hundreds of bugs and glitches in the vanilla game, such as a leveling multiplier bug that makes the Civil War quests far too easy if you are above level 10. Here is a complete list of all the fixes: http://baldurdash.org/TESSkyrim/USKPFixList.html

Arthmoor's "Live Another Life" is a brilliant mod that allows you to select a background and starting location for your character from among a dozen different possible origin stories, from being shipwrecked off the coast of Winterhold, to starting as a Necromancer, or a member of the Forsworn. You can even select a "Surprise Me!" option, for added fun with a random origin. Here is a list of all the different origins you can choose: http://pastebin.com/index/rqRqApGH

Deployable Traps and Throwing Weapons - Death From Afar combine well to add an entirely new player class to the game, allowing a stealthy ranger character to make strategic use of numerous traps to take out enemies big and small and craftable throwing axes, torches and special poison or vials of flaming oil. This is a great way to experience Skyrim combat in an entirely new way.

Build Your Own Home is the best player home mod for a realism playthrough, and Sollar's Scenic Carriages enhances the realism and immersion of traveling across the land. Carriages are now able to transport you to any other city in real time, with a cantankerous Nord driver who points out various places of interest along the route, even stopping and waiting to allow you to exit the carriage and dispatch any enemies you might encounter along the roads. Just don't keep him waiting too long, as he's liable to get ornery.

Cliffworm's excellent Sounds of Skyrim: Dungeons enhances the Skyrim soundtrack with chilling ambient dungeon sounds that when combined with Darker Dungeons, Powerfang's Deadly Traps, SM Drop Lit Torches and Wieldable Lanterns takes the concept of dungeon exploration to the next level. Sounds of Skyrim: The Wilds provides the same level of enhancement to the ambient wilderness soundtrack.

COMPATIBILITY:
I have not experienced any conflicts using these mods; however, there are some important load order guidelines to keep in mind. Unofficial Skyrim Patch should be at the very top of your load order (just after the official high res texture pack, if you are using it). Live Another Life must be placed very close to the end of your load order, and ASIS.esp must be at the very end, with ASIS-Dependency.esp just above it (second to last). Deployable Traps needs to be higher in the load order any other traps related mod (including Powerfang's Deadly Traps). RADIANCE, Occupy Skyrim, Real Wildlife and Deadly Dragons should also be close to the end, just above Live Another Life. Also, Bring Out Your Dead should be at the top, just below USKP.

These mods are completely compatible, so you can click SUBSCRIBE TO ALL if you want to add them all at once.

As is the case with any mods that incorporate scripts, it is always best to keep a clean save game on file to revert back to in case you decide you don't like the mod.

To get the most out of a survival/realism playthrough, it is advisable to tweak the game settings in order to disable the compass and slow down the timescale a bit. In order to disable the compass, you need to open SkyrimPrefs.ini in a text editor (location is C:\Users\USERNAME\Documents\My Games\Skyrim\SkyrimPrefs.ini), and change "bShowCompass=1" to "bShowCompass=0". Thanks to Whickus, we have the Better Quest Objectives mod, which enhances the descriptions for many of the quests to facilitate a more immersive way of finding quest locations.

If you find that day and night cycles too quickly when embarking on a journey, you can easily slow down the timescale using a console command. In order to slow it down to half pace, for example, open the developer console in-game and type "set timescale to 10" (the default timescale is 20, and real time is 1, so any number between 1 and 20).