That's the reason why communicating on the web is valuable; to get around the slowness of software releases to show us that you're doing something.
That can be avoided by inserting qualifiers into the text, such as the obvious "no promises". Say ideas are "interesting" or "have possibilitiy" instead of being "good" or "have value". It can also help to type up a single longer disclaimer and then link it from posts, or even from a signature.
Once that is done, any customers who express unwarranted expectations will have the reality explained to them by other users who point out that disclaimer and show what it means.
In addition, a suggestion can warrant a reply even if you think the idea is no good for DDO at all! That way the player knows that reposting it in similar forms won't be helpful (and he gets the satisfaction of seeing a developer take him seriously). When doing that, it helps to soften it by saying the idea "would have trouble working with our mechanics/budget/theme", instead of saying that it's bad overall. (For example, here we see a developer saying no to a player suggestion. That can be better than saying nothing)
It is notable that aside from Lamannia forums (which are only sporadically activated), the DDO forums includes no "feedback" section. Where should a player go if he wants to complain that Weapons Shipment is a boring quest or Dreaming Dark lacks loot rewards? To make it fit in Suggestions he'd need to come up with a way to fix it, which is a higher bar than just pointing out the problem.
Look at it from the player's perspective:
According to MadFloyd above, developers are not able to read every thread. Unless there is a developer reply (which hardly ever happens), players have no way to tell if a developer has looked at the thread or not.
Given that the player believes the idea is a good one, what other option does he have than to repeat the suggestion from time to time?
For example: the Bluff skill. A very weak skill, which players do not believe performs the game role it should have. Bluff wasn't even buffed back when the other active-combat uses of charisma skills (Intimidate and Diplomacy) were. How can a player guess if the nonresponse to suggestions regarding Bluff is because
1. The developers haven't happened to see it
2. The developers already have a change coming in a future update
3. The developers don't want it changed
Although I'm a big supporter of thread titles which indicate the thread topics, I must point out that historically on this forum, threads which call out a developer by name have a higher chance of getting a reply.