Dps
Blue
American Steele: 50 BS/Inv
Nightfall: 50 DDD
Sable Slayer: 50 DM/Rgn
Fortune's Shadow: 50 Dark/Psi
WinterStrike: 47 Ice/Dev
Quantum Well: 43 Inv/EM
Twilit Destiny: 43 MA/DA
Red
Shadowslip: 50 DDC
Final Rest: 50 MA/Rgn
Abyssal Frost: 50 Ice/Dark
Golden Ember: 50 SM/FA
I think part of the OPs question is really more about thematics than output. Basically, "why is the only set with fire's numbers only available in a fire theme." The answer to that part of the question is that is just how most of the powersets are set up. 1 or 2 (mainly Earth Control) let you change the theme somewhat drastically.
To answer the question of why you see clusters of certain character types, I think this is partially because players play "follow the leader" and tend to build the same combos that other people have demonstrated as effective. One current flavor of the month combo, Illusion/Cold, was virtually ignored for nearly a year after proliferation made it possible until one player built one and started posting videos of it.
In general, defensive sets that add offense as well offer less mitigation than the ones that only offer defensive abilities. Stone Armor is the toughest defensive set, at the cost of actually lowering your damage output. Most others are a bit weaker than Stone and offer only minor offense (a damage aura) or none at all (non-damage taunt auras). Fire and Shield both offer significant offensive boosts, but are weaker than other defensive sets at actually keeping you alive... and Fire is the weakest of all at mitigation so it's only natural that it be the best at improving offense.
(Actually, Shield was the best offensive boost until the recent Fire buff, which was very annoying to people who liked Fire Armor. Now Fire is a bit better but Shield still adds plenty of damage too so it's a perfectly valid choice if you want high DPS.)
Cascade, level 50 Blaster (NRG/NRG since before it was cool)
Mechmeister, level 50 Bots / Traps MM
FAR too many non-50 alts to name
[u]Arcs[u]
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