I think there's a sensible middle ground to be found here.
If a group is too small to complete the main quest, then the consequences are significant: they cannot earn favor and will not receive the end-of-quest reward. Faced with that prospect, most such groups would prefer not to enter in the first place, and they should be given a warning to that effect rather than discovering after an hour or two of gameplay that Turbine deliberately designed the adventure to be inaccessible for a group of their size.
On the other hand, if the group simply lacks the appropriate stats or spells to complete an optional objective, they can still earn favor as well as the requisite end-of-quest reward. Faced with that prospect, most such groups would prefer to enter even knowing they'll miss out on a bit of experience and perhaps a chest or two.
So I'm not sure it's necessary to warn groups that they'll lose out on a 500XP optional objective if they can't activate a strength rune, for example. But we can and should warn 1-man and 2-man groups that certain quests simply aren't designed to be beatable by them, especially at a time when Turbine is trying as hard as it can to make DDO more solo- and small-group friendly.