Are there any missable quests?

Too many to list here. Any quests from Lothering or Ostagar are missable, as there comes a point where you can't go back to them (you can still get itesm from Ostagar though). There are lots of places that you can only go to once, during a quest. Some items are also highly missable and fairly obscure. Your best bet is to do absolutely everything as soon as possible.

If you complete the landsmeet it is also possible to miss quests or be unable to complete quests that require redcliffe.

If you go to redcliffe do NOT leave once the guy asks you for help against the attacks. If you do and then come back to redcliffe, everyone is dead, you miss out on a few quests + the best longbow in the game.

And what is that longbow ?

what longbow? the best longbow to my knowledge is like fetches reach or something that is in the chest behind the dragon that is in the "nature of the beast" quest. You get an even better bow for lilliana (spelling) if you do her personal quest and kill her mentor... what longbow are you talking about...

Far song

I didn't know the game was like that... Wow... I thought (Hey RPGs don't work that way!) Good to know actually... Also I don't remember a Longbow! and I beat the game twice! If I'm missing this bow somewhere I require to clam it!

There are millions of quests you can miss... Mainly places you can't go unless a quest allows it. Also if you not careful you can miss quests that will not show up properly and some quests are skill based... For example if you're a rouge and go to Denerim some guy will make requests for thievery. I know this because I first played a Mage and he was not there... I could be wrong but from the looks how this game is put together is that the game's quests are sometimes based on what your character is.

All in all like most RPGs quest can be missed quite easily.

Slim Couldry (the Denerim guy) isn't entirely missable. He merely requires the Warden to have a rank in either pickpocketing (which any Warden can take) or Stealth (which only Rogues can take.) You can, however, miss out on some of his quests if you choose the option of "I'm good at pickpocketing, but not breaking in" (or something like that) and vice versa. Just say "I do it all" or something like that and use Zev or Leliana to do the pickpocketing/lockpicking if you have to. The break-in jobs don't exactly require a lot of stealth.

I have found, though, that not being a rogue sometimes means missing out on things near the beginning of the game. Not that the stuff's that awesome, but there are locked chests in Ostagar that Daveth can't pick, not to mention ones in the Beacon Tower, too. And you can only return to Ostagar if you have the expansion, in so far as I know, at least on the PS3 version.

MARJOLAINE'S RECURVE is 1 of da best bow dat can be found

Don't talk to anyone but Shopkeepers and Plot related NPCs. Don't venture into areas unless specified by a Main NPC. You will miss all the needless quests.