1. I'm not actually sure what the game looked like 5 years ago, but I'll try:
You may have been before the Great Stun Nerf, making Smuggler's bonus damage somewhat less reliable.
More levels have been added, making it possible to out-level encounters you couldn't before.
XP to level past 5 was increased.
Tech Specialist Volley was replaced with Charge Weapon, which is the best melee buff in the game.
Wings, Jetpack (accessory) and the Lunge perk (level 10 Mercenary) allow hitting Flying enemies with melee attacks.
Tease skills are gone, now effectiveness is based on Libido.
Ice Cold character creation perk makes your Tease attacks more effective with low Libido, although you still want +Sexiness gear for more damage.
Clothing/Armor now always has Sexiness, Resolve or both. Sexiness adds to Libido, Resolve to Willpower. Outer clothing has up to 5 total, Upper and Lower undergarments have up to 3 each. Unoccupied slots count as max Sexiness for that slot. Resolve and Sexiness add 1/2 their value to max Willpower or Libido, respectively.
There are now Tents that allow you to Sleep (and level up) in hostile areas with a cooldown. You start with one.
Shekka on Tarkus now sells an intermediate shield between the Decent JoyCo and the 'Hammer' from the Sex Bots. For that matter, the Basic JoyCo has slightly more Shields.
Ships have storage, and so does your apartment on Tavros in the Nursery second floor.
Taint was added. You get it mostly from illegal Transformatives and Alien food. It decreases by 1 72 hours after it last increased. It increases minimum Libido. If it reaches 100, you will never be able to decrease it with any in-game means.
Sex no longer has a minimum Lust requirement.
2. In combat, you can Attack, use Gadgets (1/installation/fight), Charge (Energy Regen x2), or Evade (+50 Evasion). Firing weapons consumes Energy. Each turn you regenerate a certain amount of Energy, even if you don't Charge. In general, you want to Evade when your enemy has high Energy for their guns, and attack otherwise. At least, until you get a good enough ship to easily win. You can get a better ship for free at Zheng Shi, but you lose your current Ship. If you want to keep the CassTech, you need to buy a different ship from Vahn on Tavros. Better ships have better stats, and more storage.
There aren't many Ship combats in the game, yet.
Random: Casstech Z7 (designed to be beatable with the starter Casstech Z14) and a Spearhead with a Cloaking Field.
Set: Breaking into Zheng Shi (avoidable by recruiting Paige, rescuing Kiro or 'guessing' the password), come back to Zheng Shi after stealing the Sidewinder. Avoidable by not being in the Sidewinder.
3. You can avoid sex with most NPCs. Just don't lose combat, which isn't too bad so long as you keep your primary attack attribute up.
If you want to rescue Kiro without fucking her, you have to be a Tech Specialist.
Shizuya's encounter is annoying, but you can beat her up later for the best melee weapon in the game, the Custom Shock Gear.
Bothrioc Quaddomes no longer appear unless you've started their quest.
There's a good chance you'll want to pick up the Sleep Fapnea on Myrellion. Make sure to set it to 'Chaste'. Some dreams are gated annoyingly loosely.
Edit: Don't give the Sydian C-Boy his dick back if you don't want to fuck him.
