As usual, Robert is exactly right. The only thing I'd add is this:
Quote Originally Posted by naaros View Post
3) For some reason, I'm getting less EXP/Skill points from mobs now that I'm 21. Is this normal? I was fighting some level 28-29 enemies in Coerthas (was exploring - beautiful zone), and I only got 400~xp and 240~SP. Before, when I was fighting mobs 8-9 levels higher than me, I would get 1000~XP and around 500~SP. Am I doing something wrong?
Robert's correct that you get a bonus to XP/SP at lower levels. At level 1, they take whatever XP/SP you'd normally get, and multiply that number by 4. That multiple decreases until you hit level 24, at which point you get the normal XP/SP for the mob.

Something else to consider is that different mobs give different base XP/SP. So, for example, if you kill a puk/djigga/coblyn that's the same level as you, you'll get 63 SP. If you kill a raptor/basiliks/ahriman/salamander that's the same level as you, you'll get 150 SP.

If you want to get the maximum XP/SP per mob, you should fight mobs that are 10 levels above you. You get more XP/SP for fighting mobs above your level. But that maxes out at 10 levels above you. So if you kill a raptor that's 10 levels above you, you'll get more XP/SP than if you killed a raptor that's 2 levels above you. But if you kill a raptor that's 20 levels above you, you'll get the same XP/SP as if you killed a raptor that's 10 levels above you.

Finally, you get slightly less XP/SP if you're in a party of 3 or more people. But you kill things so much faster that you end up getting more XP/SP per hour.
5) As CON, is it worth putting the DoTs like Shock, Frost, etc on while soloing or should I just be spamming my nukes? Currently, what I do is use Profundity + Spiritbind + Blizzard to open with. Then I put on Shock, and use Thunder, then Blizzard again, etc etc. Is there a better way to fight as CON?
Like Robert said, DoTs are great against NMs and high level mobs with high defense. That's because DoTs cause a specific amount of damage per tick regardless of what level the mob is, or what the mob's defense is. So if you have Piety at around 75 and cast Poison II, it's going to do (about) 63 damage per tick to a level 1 mob, and it's going to do 63 damage per tick to an open world NM that's above level 70. That's substantial because every other attack in the game does less damage to higher level mobs. And when you get to open world NMs, a fully buffed melee weapon skill will sometimes do about the same damage as a DoT does every tick.

In my opinion, when you're fighting mobs around your level, nukes seem to do more damage per cast. But when you get to high level NMs and boss fights, DoTs are huuuuuge.

Glad to hear you're having fun. And welcome to the game.