Scimitar dance
Cheese a win if you're counting on missing. Definitely pick it up if your build qualifies you for it anyway (yes, it's good), but it's not worth building for.
HBO
Once again, pick it up if your build qualifies you for it. At-wills should be better than basic melee attack, after all. Builds that rely on other stats should not gimp their primary for the str and dex requirement.
On some builds, such as a brawny rogue, it's made of awesome.
all opportunity feats
Which ones do you mean? Blade opportunity is one of my favs on a fighter, because a fighter's OA is just plain better.
I don't bother with most other builds.
Now, for most games, you can't count on getting OAs ... but with crazy high-level tactics such as abusing daze effects and readied charges (to trigger on a dazed enemy's charge), incidental OAs are a lot more common.
all critical feats
Depends entirely on how much you're rolling. You'd be epic tier, after all ...
And if, say, a fighter/swordmaster chooses mostly close burst 1 powers, and has both of the ones that force all nearby mobs to move in, the sheer number of rolls = a fair number of crits.
Now, the crits themselves aren't that great, but the swordmaster gets a power back when he does ...
Now, those feats are nice to have, but are hardly the best.
Nimble blade can be @#$%! if you're tactically minded (and thus get CA usually), and you use a light blade. Like ... oh, a rogue.
Spell focus may be largely wizard only ('cos MC wizards won't have that many wiz powers), but a -2 penalty to saves for all powers is @#$%!.
My most recent fav underrated feat (since superior crossbows in AV):
Steady shooter. Really.
It gives you +3 damage (stacks with everything) if you haven't moved since the end of your last turn.
Shoot before you move every turn, and you're golden.
If you're sticking to the fighter like glue, provoking OAs could get your foe whacked by the fighter, so it's not that bad a deal.