The quickslot penalizes the resource cost for the skill, iirc. So using a hotkey for a skill that has a kb input combo but also says in the description 'Quick slot available' or 'Can be used with Quick slot' costs more mana/WP/SP whatever. It's REALLY negligible tho, and doesn't seem to apply on everything.
I just did some testing to see if I could figure it out for you and on my warrior using Ground Roar there was no difference between using a hotkey or kb input. On my ranger lifeskill alt, Will of the Wind II cost 45 mana for Shift+Q and 50 mana for hotkey. On my wizard, the pet summon cost 100 mana regardless, but the awakening meteor cost 210 for hotkey and 200 for Down+F. So it seems to be like 5-10% of mana cost. However I don't think you can cancel the animation of other skills with a hotkeyed skill, and I know for sure you can't use some of the Ultimate ranks of skills (which are often the cancels you want) with a hotkey - which doesn't matter much in PVE but can make a huge difference in PVP. Certain classes rely pretty heavily on animation cancels and flow skills to perform well.
I was thinking back to what was really my driving force for quitting this game. I know the gear was - but have they loosened up on hidden stats at all? I.e. luck?
They have not loosened up on hidden stats. There has been some datamined info released recently about how AP/DP actually affects your character (basically plays out as AP is a combination of damage + accuracy and DP is a combination of damage reduction + evasion - and different items have different ratios of those sub stats) but much of the actual information relating to combat is still very much hidden, and there are some things that are handled server side and can't be datamined apparently.
Luck is displayed on your character sheet and caps at 5, which you can get pretty easily with the underwear and title benefits. If you mean individual character RNG then I have no idea. Some people claim that certain characters are luckier than others but I don't really buy it. I failstack across multiple characters and I haven't observed anything abnormal. I've failed TRIs at 63 failstacks and upgraded TRIs at 21 failstacks, I've gotten +14-+15 without any failstacks and failed my Kzarka +15 over 90 times at 25-45fs range before I got fed up and just forced it. It really is just RNG when it comes to upgrading.
For loot, you can use a kamasylve blessing and I think some buff scrolls to increase item drop rate chance but who knows if that means high value accessories, crystals, summon scroll drops or if it just means more trash loot. The same goes for knowledge rank on the mobs, and I believe it's been tested that node investment does not improve drop rates (or anything, really).
Boss loot is definitely tied to damage - it is
possible to get a big ticket drop by hitting the boss once and AFKing, but you are much more likely to get boss loot by being in the top damage bracket. Most of my boss drops have been from going nuts on a boss late at night or early in the morning when there aren't 40+ people there, fully buffed and sticking to the boss' back with an assassination elixir with a clean rotation of my big damage skills. This may be why you saw more loot from your ranger, especially if you quit before awakenings were released. Pre-awakening warrior was absolute trash and probably the lowest damage output in the game whereas ranger was solidly in the top 3 and had the benefit of range to avoid many of the mechanics that prevent melee from constantly hitting the boss.