I love a bit of concrit. I am (I hope) a competant writer, but I recognise that I occasionally loose the plot (and the narrative, and my grasp on characterisation, and any sense of perspective...) and need someone (that would be you) to point this out to me. Concrit seems to be something of a dirty word in fandom; something to be done in private, lest someone's sensibilities are offended, and that's fine, because, as you point out, most people read and write here for fun. I like concrit because it gives me an opportunity to make my writing less crap, and this can only be a good thing. With this in mind, I've decided that the more betas, the better, because it is so rare to have someone offer concrit after a fic is posted (and even then it is often really minor quibbles that can be answered by saying 'if the Doctor doesn't trust Jack with a fully-functioning wriststrap, I sincerely doubt he let him keep a key to the TARDIS', or 'Teddy doesn't make a personal appearence in this story because he is away at boarding school in Scotland).
Finally, concrit is like croquet; not something one ought to indulge in while drunk on bubbly at 8am on the front lawn of one's University campus, then (you may recognise the Voice of Experience here).
*makes mental note*