re: Dipping
Dipping is not a problem for me and my gaming groups (of which I have about 3, 2 in NYC and one in FL). The biggest reason is one you hint at: Fighter 2/Paladin 4/Thayan Knight 4 is a whole lot weaker than Druid 10 or Conjurer 10. Another reason is that we view classes as necessary abstractions (though mucking around w/ them is part of the fun of D&D), so "fighter" or "kensai" doesn't really have much independent meaning to us unless you want it to. So, sometimes you just want the 2nd level ability from a class, and that's fine.
Implicitly, I think we do demand that people make characters. They need some flair and some individuality and their abilities need to -- more or less -- hang together in a sensible way. I cannot recall the last time that was an issue of any sort, though. People just seem to naturally do it.
re: Ending Fights
If that's someone's character concept, then it's the sort of thing the DM and the party has to roll w/, assuming the character isn't broken. And, we have our own sense for what that means. For example, one of my good friends was playing a Beguiler for a while, and he had high DCs and he routinely either ended or mitigated fights. That was his bag. Well-placed Illusory Pits and Hypnotic Patterns cut the enemy's numbers in half. On the flip side, when they did save, nothing happened, and that was something he had to deal w/. So long as the save DCs aren't too nutty, and the player realizes that sometimes his save or suck abilities won't work (e.g., golems), I think things are fine.
I think I might be less sanguine about it if it wasn't part of the character concept or if as a DM I was somehow blindsided by it. But, my group knows that this is a big part of what say a Beguiler or a Conjurer does. We also do go out of our way to tell the DM what our characters can do, since he or she typically has better things to do than comb over our builds.
I'd also say as a player and DM I'm pretty comfortable throwing another wave of bad guys if the first one gets mowed down too quickly or in undramatic fashion. The reverse is true, too. One of my DMs recently cheated, but in an interesting and story-driven way, so that we'd be in a little bit better shape for a second encounter b/c the first one was harder than he anticipated. Note that this wasn't during the fight -- he essentially gave us an easy healing opportunity for one of the characters.
EDIT: ninja'ed by the OP. I don't tend to think of classes as anything other than related groups of abilities, though they are sets of abilities that do at least capture some concepts. But, when I hear "barbarian" I hear "tankish melee combatant w/ ability to boost stats and wilderness skills" or something to that effect. "Sorcerer" is "spontaneous casting arcane class." Stuff like that.