Stronger characters fill a needed role, stronger characters fill that role as efficiently as anyone else in that role, stronger characters hit their stride with a reasonable number of ability selections from their skill tree. Stronger characters can perform well in all difficulties.
Weaker characters perform a role made superfluous by other characters, weaker characters perform a role but take longer to do so or earn less XP for the team doing it than others would, weaker characters need many skills and passives deep in their trees to become effective. Weaker characters may perform well in lower level difficulties but weaker or very poorly in higher level difficulties.
Alchemist, Reaver and Templar in that order need the most help.
Most non-Mages need a tweak if not help.
Warriors
Legionnaire - Strong survival, low damage, low CC - Best tank, playable in Perilous, sometimes made superfluous by strong CC and damage, sometimes made superfluous by the over performing ultra high survivable medium damage Arcane Warrior. Would benefit from increased XP generation, would benefit from scenarios that require aggro management and survivability. Needs to have key skills such as Walking Fortress and/or Counterstrike within easier access
Templar - medium survival, high combo damage, buff support - needs an assist with guard generation. Needs a viable non combo based build. Hard to justify in Perilous.
Katari - good damage, good survivability, decent CC - recent tweaks to Katari seem to have nudged this class to a nice playable sweet spot in the rankings, fun style, effective CC with knockdown skills and with Guard on Hit can survive to stay in the game. No changes needed
Reaver - Strong Damage, low survivability - Live on the edge gameplay can be fun but doesn't transition well to Perilous, needs survivability tweak for high levels
Rogues
Archer - Strong Damage, low survivability - My favourite class, gets critical skill early, deals damage like crazy, has to rely on the team to stay alive, it could use some tweaks that encourage the use of more than Upgraded Long Shot. Full Draw is too slow and only used while learning the class then never used. Easily playable in Perilous.
Hunter - Good Damage, medium survivability - good class, but Stealth without I Was Never Here is a tough sell. Lack of synergy across the skills but enjoyed by many due to some fun skills. Would benefit from synergy that would allow it to compete with the best classes.
Assassin - Strong Damage, medium survivability - Amazing single target damage, multiple powers have targeting/tracking issues, steep learning curve to perform at top levels but ultimately viable.
Alchemist - medium to good damage, low survivability - Weakest class in game by far. Fun skills that can't finish the job and leave the class exposed in melee range and dying. Difficult to play in Perilous without judicious amounts of skill or promotions. Luka to Luke, "Why did that take so long??"
Mages
Keeper - low damage, good survivability, good CC, strong buffs - As long as they are handing out Barrier, many ways to build and play, works in all difficulties, still needs a team to help out with damage. Excellent class design, especially with the immediate access to the key class skill, Barrier.
Elementalist - Good damage, good CC, good buffs, good survivability - Lots of options in the class, does many things well but not so well that it makes the team meaningless, no changes needed beyond a limit on Fade Cloak
Necromancer - Good damage, medium CC, medium survivability - fun assortment of skills, no viable barrier makes surviving more challenging than other mages but self rez options are an interesting twist. No changes needed other than the missing skill than summons a Skelly Horde
Arcane Warrior - medium Damage, Ultra survivability - fun class with immediate access to the best skills. Problematic because it needs no one else and in fact performs even better if played selfishly never waiting for the team or leaving them in the fade.