Monsters, lots and lots of monsters.  They are literally oozing out of the cracks between every block.  The largest threats you face are the mobs from Advent of Ascension and Lycanites' Mobs.  Spawns are highly biome specific so the location you choose to set up shop in has a large effect on your chances of survival.  

You will want to set up your first base in a forest or plains type biome.  Avoid savannas, deserts, and jungles. Savannas have hunter mobs* that you cannot hurt at the beginning of the game.  Deserts have sphinxes which are immune to melee damage.  Jungles pose a more insideous threat,  beware of the stingers.  One hit from them will inflict you with poison 4 and will drop you down to one HP in about 3 seconds.  Also, bush babies make the most annoying noise you have ever heard.  You don't want to be hearing this non-stop in you base.  Tiagas are acceptable, but infested with ursas.  These are bear type creatures with deep health pools, a strange walk cycle animation and the tendancy to be completely silent.  They will sneak up on you when you are outside performing menial tasks around your property and can deal some pretty hefty damage to an unarmored player.  This threat is more managable than the others mentioned here so a tiaga can still be considered a viable biome to live in.  

Beware of world events, I won't ruin the "fun" by listing them out here.  Just know that they are a thing, and they are all bad.  World events can happen during the day or night and will be announced at the start and end if the event, they all involve extra spawns of mob types that (usually)don't occur outside of the event.  There is also a full moon event on every full moon (every 8th day), and a monster raid every 7 days.  During a monster raid, mobs will actively attach your base, both placing and destroying blocks to reach you.  They will also try and kill your livestock.  Sleeping through all of these events, or hiding in a bunker away from your important stuff is recommended until you get some good gear and automated defenses.  Another wise technique (not seen in the corresponding let's play) is to have your early game base embedded in a mountain, or underground.  This will give you the best protection from some of the nastier spawn types and their explosive personalities (Bonus Hint: GregTech machines respond poorly to booms).

One final thing to be aware of is exposed lava/fire.  Really nasty things spawn in lava so you don't want to build your home next to an exposed lava pool because you think it's a valuable resource.  Seriously, if you are exploring and you see an open lava pool you should be on high alert.  Smaller flows will not spawn the lava mobs, but both source and flow blocks count toward the total amount of lava required to be considered a spawnable location.

Once you have chosen a suitable location, it's time to use the crystal ball that you started with.  This is provided by Minecraft Come Alive and will provide you with a free house.  The house will take up about a chunk of space and will carve out any blocks that are in it's way.  It is highly advised that you take advantage of this opportunity as the house comes with a bed, so you won't need to worry about finding sheep to be able to set your spawn point.  When you use the crystal ball, you will be teleported to a small room with an enchanting table.  Click on it and you will be asked some questions like your name, your type of naughty bits, the type of naughty bits you look for in a partner, and what your living arrangements are.  You can choose from "I live alone", "I live with my family", "I live in a village" or "Other".  To properly adhear to the the packs "cannon" you should choose "I live alone".  The family house is inferior to the solitary house and living in the village runs directly against the pack lore so I can't in good faith advise you to do this.  This option also requires a very large space to spawn correctly and it can really maul the landscape.  Choosing "Other" is a terrible option as it gives you nothing so choose "I live alone" for the best results (Note: This will generate an above ground dwelling which we determined in the previous paragraph, is a bad thing.  This is your initial shelter, you should get to work on creating a proper fortifed encampment as soon as you have food and tools).

Finally, don't be afraid to be choosy with the map you are starting your play through in.  You should make sure that creative mode/cheats are enabled when creating your map so that you can fly around for a bit to look for a proper location to /setworldspawn.  If the local area doesn't have any decent locations to start your playthough in feel free to roll another world.  You will be playing this pack for a long time so don't feel bad about being choosy.

As a side note about worlds.  If you are experiencing lag when generating new chunks, the chunkgen mod has been included in this pack.  You can run a server copy of the pack and use the command §0§l/chunkgen <x> <y> <height> <width> [dimension]§r to pre-generate as many chunks as you want ahead of time.  Then just move the world back to your single player client, or play using a local server.  Whatever works best for you.

§0§o* Hunter mobs are an AoA concept.  They require a certain level of hunter skill or you will be unable to damage them.§r