Your character moves to Legacy and that’s it. Same game, same content, you can continue playing like nothing happened.
What changes is …
- Your stash tabs will be lost.
Think of it as having two separate stashes, one is for Cycle and one for Legacy. So your Cycle stash contents are moved to Legacy and become a dropdown that’s “remove only”, so you can still access everything you had, but you can no longer store new stuff in there.
You can store any new stuff into your Legacy stash, but if you never played Legacy, your stash will have one stash tab = you lose your stash tabs 
- uhh, Bazaar will now list items from people who played a year ago with weird and exorbitant pricing. If you don’t play Merchant’s Guild, no effect on you

- you’ll see less people in hubs since most players will go play the new cycle/season
Yes, but you have to go to your stash and press a button for … reasons unknown … to get back your crafting materials and gold and faction reputation. No clue why this isn’t automatic.
There’s no specific DPS number, but basically if you can kill him without dying and it takes you less than 3 minutes you can consider your build to be very good 
3000 HP is a good starting point for melee. As you obtain better gear, you can go even higher, some players have 4000 or even 5000 HP, but this may be a tradeoff for less damage.
Armor is a form of damage reduction, so it’s always good to have it rather than not. Around 8000 Armor is a good number to target, you can still go higher (players stacking Strength often do) but the diminishing returns start to kick in.
More importantly, be sure to get some kind of healing too - it could be a spell that directly heals, it could be HP regen, or it could be Health Leech.
Swiftness isn’t compulsory, many people play without it. But you are right that it’s very useful, moving fast helps you avoid damage from bosses and makes the game feel more ACTION packed 
With the addition of Evade, I think you don’t actually need a ton of movement speed, but comfort level is subjective for each player, so let’s say having 40% movement speed should be the bare minimum.
How to approach this depends on your class and build. Some classes use Ward to boost their HP total. Some choose to take the hits, healing through them. Some can get “tanky” and reduce the incoming damage a lot. And some choose to go crazy for DPS and try to kill bosses before they can do their attacks 