The reason for having insta cast GCD spells is so you can move - that does not exclude them from being used when standing still though but they are there to help out casters when they need to move. Off-GCD skills generally have a cost associated to them outside your primary resource - be it a long cooldown(virus) or resource usage(aetherflow stack for energy drain).


Also there is skill queueing if you don't use macros. You can hit the next skill when at around 1/4 gcd left. So instead of tiwddling there waiting for gcd refresh after sustain(this is really only relevant in solo content or other odd cases - and it doesn't require to target the pet to cast it on them) you can already get read to queue your next spell.

2. Tab targetting as said is very reliant on your viewport - usually angle up the camera so you are staring down at yourself can make usually tab targeting very much easier.
3. Used to be you had to dismount to speak. There was no auto dismount. And really is it hard to see the immersion of this? You're an adventurer not a lord or lady.
4. Phais already said it - some debuffs cannot be removed as well. The icons generally are very distinct as well.
5. The armory will grey out things you cannot equip due to your level - outside of that anything that is not greyed out you can equip on your current job - the one exception here are weapons because weapons define your job - you equip any other weapon than yours and you will change to the job that that weapon can use - if you unlocked it that is.