We are casual gamers and we play every other day or so. Because of children and RL we find it difficult to do many raids and epic quests, so our chances of making GS or epic upgrading items is slim. Yet we've managed to cap a toon each and are well on our way with a dozen more.
Overall difficulty? It seems more or less accurate until level 12. Some of the quests after seem to assume that we have GS/raid loot and they are more challenging. It was at this level that we started using hirelings.
Inconsistency? If the quest is an extreme challenge, they warn you before you enter. If you are not geared for an extreme challenge don't go in. There's your easy button.
Challenge vs. success? I feel that there should always be a chance of not being able to complete a quest(tank commanded, healer stunned, big trap before the first shrine blows up on the rogue, etc.). On normal there should be 95% successful completion at level. Hard, more like 85% and leet should be 75%. With farm geared twinks, add 10-20%. There is always a chance for one of the key characters to miss a save and get knocked down, stunned, or dead right at the wrong moment. Epics should be 50/50 but their drops should increase.
45min frustration? As many clerics will tell you, you can't fix stupid. Some players will attempt to zerg and wipe in the first room, then act like it's the clerics fault. Some will be over confident going into the final fight and 'DING'. On a few of the quests, there is a mid-quest fight that seems harder then the end fight, maybe on upcoming quests, there can be an early fight that puts players on their heals and makes them run that one more carefully.
Balance? IMHO, it just can't be done. As it stands now, melees can trot through the first five levels on DDO, while rogues and casters are a bit more squishy. The next five are spent looking for any and all bane weapons by a melee and casters get real DPS spells but still have to watch their SP. Levels 10-15, casters get CC, great buffs, major damage spells, and SP gear while melees begin the never ending farm for DPS gear and carefully pick and choose feats, enhancements and Pres. By endgame casters can instadeath multiple foes, summon IX, debuff baddies, and let's not forget GODmode(for some). While melees, who require buffs, heals, and a splash of something else(for that feat that they just don't get), will still be farming that DPS equipment, getting vorpals nerfed, having to break fortification, and their AC is made irrelevant, But they have alot of HP(they'll need it).
I've been a PnP gamer for almost thirty years. I've made characters for every class with almost every combination. I've made every fun/tweeked/minmax/gimped build just to see how it would play. I've continued doing so on DDO with roughly 20+ toons(total) spread across 5 servers. Barbarian halflings, Bard horcs, and toons based on comic books(Hellboy, Ironman), I'm here to not fit in the cookie cutter and to have fun.