Useless feedback is when a tester points out something as a problem that is only a problem to the tester because they don't understand the game mechanics. Ex: The game needs cities and roads to be good. There has been quite a bit of this and the only thing it really tells the devs is that the tester did not play the tutorial.
It is also useless to develop convoluted solutions to problems in the game. Ex: Magic is too rare, don't make it require mana. Techs are boring, don't have any techs. Obviously there is a set design for the game. They are not going to change core features that are present in every TBS since MoM at the whim of a few testers. But even this tells the devs that techs are perhaps unbalanced. Reporting the problem is useful. That should be the focus of a beta tester. Go ahead and think of possible solutions, but don't expect the devs to necessarily implement them all.
Useless feedback can also be in terms of the soothsayers that are still pissed about WoM and are sure that this game will suck too. There is a clear vision and scope of this game. It is not AoW:SM, Mount and Blade, HoMM3, or a clone of MoM. It is loosely based on MoM. Take some time to learn the limits of the engine and what can be done with the modding freedom we have. You won't be able to add a siege function or LOS. You can certainly make larger more interesting tactical battles, spells, abilities, improvements, techs, Paths, traits, races, factions, Sovereigns, and monsters. If that doesn't satisfy your TBS needs, the game you want this game to be exactly like is the game for you.
There is alot of good feedback on the forums too, but with the good, comes the bad and the ugly.