I have been in terrible games that allowed every book/supplement published, and I have been i terrible games that were Core Rulebook Only.
First - is the DM having fun?
Second - are the players having fun?
If the answer to either of the above is not 'Yes' - then keep asking 'Why?' repeatedly until you get to the reason(s).
DM Example:
GameCounselor, "Why are you not having fun as a DM?"
DM, "Because my min-max'd, munchkinizing, powergamers just walk through every monster!"
GameCounselor, "Why does that bother you?"
DM, "Because the players are not being challenged."
GameCounselor, "Does that bother the players?"
DM, "Dunno, probably not."
GameCounselor, "So why does that bother you?"
And so on... and so on... -sip- etc...
What concerns me most as a Player and DM, is when one/some of the players are lower/weaker than another/others.
Party Power Balance is KEY!
As a DM, that is when you tweak events/things to help give the lower one(s) a little something extra to bring balance to the party.
As a DM I am never concerned with a Party being too powerful - you can ALWAYS boost monsters, make harsher traps/environments, multiply monsters... heck - bust out an enlarged Mirror of Opposition to have them fight themselves.
NOTE: As a DM, never try to weaken the PCs, take their stuff away, or mess around at character creation - THAT just demoralizes everyone!
-whew-
Enough Ranting...
Thank You for Listening