There is also the possibility of plateauing. While the UFP might look like a veritable think tank in combining the innovation resources of 150-something cultures, it might be that each of those cultures had already reached the same peak level when joining (else how did they survive the Klingons before joining?), and there's no major synergy advantage from 150 plateaued cultures knocking their heads together.
Advancements do not just stop.
There are ALWAYS new things to explore and technologies to develop.
The apparent slow-down of technological discoveries (mainly being stuck on Warp drive for example for a long time) was the unwillingness/inability of writers to do it (along with other things).
In the midst of very advanced techs that were developed as a 'one time thing' (never to be heard from again), these breakthroughs could have been used in other areas (and it wouldn't take a long time for the Feds to implement it).
The main problem was the writing staff that seemingly had issues thinking within the terms Roddenberry laid out (Humanity as a culture underwent radical changes, which at the time that to a Capitalist mind-set of the late 20th century would seem ludicrous at best).
One thing some of you didn't take into account is that it seemed in early TNG, the Federation was sharing its technological breakthroughs with other races (not part of the Federation) openly.
The Klingons probably became a space-faring race because of the Hur'q some time ago given what little we were able to piece together, however, they are far from stupid, and they WOULD in effect create certain advancements of their own over time.
They would start off with learning how to use the technology and then analyze its inner workings and reverse-engineer it.
Them essentially retaining their warrior-like mentality is probably the vestige of an era that didn't include social changes.
They were conquered before they had a chance to sufficiently develop and change their culture in the process.
The Romulans had the same issue.
They broke apart from the Vulcans who underwent a large social change of global proportions at a time when Warp drive was first developed.
If Humanity launched that Warp missile before WW3 happened, chances are, they wouldn't underwent large cultural changes that Roddenberry laid out in the first place.
Humanity had 600 million dead - which included leveling of most major cities and few governments left.
The conflict essentially made a clean slate, cutting off the main root of the problems temporarily which paved way for those who advocated change (that, and seeing what happened, probably made it clear to others that doing things as they did before was unsustainable - couple that with arrival of the Vulcans, etc.).