I have a Veteran Rank 8 Sorceror and a Veteran Rank 4 Nightblade. Both are tanks. To me, the only reason levelling the Nightblade was any different was because I already knew what I was doing with my build the moment I started the character and I knew exactly what armour to build myself. If I had started clean from scratch there would have been no difference.

I think the playstyle you have (Stamtank, Magitank, StamDPS, MagiDPS, or Healer) is what matters when it comes to difficulty levelling.

There is no difference between characters other than how you build them. There is no difference in how characters level, all characters earn equal hp, magic, and stamina with each new level. This increase happens whether you put attribute points into that category or not. When you apply attribute points you are building your health and primary resource. Gear, skills, weapons, crafting, and eventually champion points all determine how your character will perform. At rank 1 all characters are even with the exception of what class skills will be available to you. But as you level and explore you will unlock world skills that are available to all characters. From level 1-15 you will only be able to select 1 weapon and 5 skills (you unlock ultimates at rank 12 of class skill lines, and rank 10 of world skill lines) then you will be able to select a second weapon and an additional 5 skills and a second ultimate. How you mix and match will determine your overall effectiveness.

Just to add to what hardcore gave you, the differences lie in the specific class abilites. But as you delve into each class, you'll find that each class overall has skills that fit most situations quite well. it's mostly about how they approach those skills, and when you can acquire them.

Example: at early levels sorcs get access to a group of spells (not suprising) that blow things up, freeze them, knock them down, etc. not much in the way of healing (although there are a few passives to get some health return back, but meh). as you level them, they gain more access to defensive buffs...and they are one of the few summoning classes (they have inherent summoning skills). many sorcs carry a restoration staff for healing early on because of this

Now as a templar, you will have access to healing skills inherently from the start. but their ranged casting is somewhat limited early on. this is easy to balance out with a DPS based staff, or even run with a 2 hander and mix up casting and melee skills.

Keep in mind that since every class can use any weapon, and each weapon has its own skill set. most class weak areas can easily be offset with the right weapon. I'm just giving you the above examples as to how they feel different in play. overall though, you will be able to build pretty much whatever you want with the right combo of weapons and class skills. outside of that, it's mostly flavor. Nightblades have cool dark red blood sucking looking skills, templars pretty much throw globs of holy light and whatever, sorcs cast elemental spells, yada yada. Dragonknights light things on fire, breath fire....fart fire...light themselves on fire...