It looks to me like we largely agree, except that you like to emphasize the average (bad) player as being the class' expectation, whereas I think its more useful to discuss what the character can/should be doing.
I'm not convinced that between blade barrier, cometfall, destruction, and Divine punishment, the cleric is really giving up a whole lot of destructive power compared to the barbarian or fighter in the course of a typical quest.
You are right that good play is uncommon. Many players come from other games that have very strict role division and expect that to apply here. Neither of us have played those games, so maybe that's an advantage to us. In a classic MMO trinity game, the tank and the healer are pretty close to incapable of doing meaningful damage, so any time they do anything except defend or heal (respectively), they are weakening the party. Players used to that come here and think that's how it works in DDO.
Good players know better. A good melee should be using CC (stunning blow, trip), debuffs (improved sunder), have pots for removing status effects, and made some provision for self healing (usually SF pots at the high end, other pots earlier), and have an assortment of buffs from clickies.
Is that the norm? No. But, to be fair, there are plenty of clerics who think they are pulling their weight by hitting Mass Heal every time it comes off cooldown, which isn't exactly complex gameplay either.
I'm not going to get into what particular players can do. In my experience, top players tend to play well with a wide range of classes because its more about wanting to be the best than about DDO having any tasks that physically complex. Whether an individual is better at top quality twitch play or better at watching the big picture of 6 or 12 status bars, is not really a useful thing to discuss. It doesn't say anything objective about the class, just about the person's interests/skills.