One problem I've always had is that the Federation is suppose to consist of 150 member worlds (at the moment I'm running on the assumption that these are 150 different aliens). This always made me wonder how then a single alien empire can even match the power of the Federation. Even if we assume that the Federation has 20 billion human citizens and the Romulans and Klingons have similar populations, what about the billions of Vulcans, Androians, or Tellarites. The combined power of four worlds can't stand up against the Romulans or Klingons?
Enterprise helped clear that up a bit (by making Earth, Andoria, Vulcan, and Tellar minor powers), its still obvious that when we reach the 24th century that the Federation is a super power (if not the super power of that region of space). At least technologically and economically.
The Federation seems very dedicated to peaceful pursuits, and just may not have the stomach for large scale war. Even when we get to the Dominion War the Federation has developed a policy of rotating ground forces off the front after only 3 months (while for example with the war in Iraq and Afghanistan today, rotation is a year. And World War 2 was the duration, with some people being in war for years). This policy is obviously not shared with the other major powers, with each power having dedicated military forces while the Federation has elements of Starfleet fully dedicated to military pursuits.
The thing I always wanted expanded on in the show, and I don't even feel the books have done a decent job of that either, are the "members" of the Romulans and Klingon empires. It was sorta referenced with the Cardassians (I find it hard to believe that Bajor is the only planet they ever conquered). Obviously these planets aren't partners like within the Federation, but a single species cannot muster the resources to take on the combined resources of 150 worlds. Even if the Romulans controlled 150 worlds, spreading even 50 billion Romulans still wouldn't match up to the populations of 150 individually growing alien species.
But then again this is a TV show. We are suppose to believe that a plasma torpedo fired at the Enterprise can accelerate to Warp 8 and not have hit the Enterprise instantly after being fired.