It affects the whole raid, but it is equivalent to windfury totem. So not really worth worrying about (pretty far down in frost tree).

The thing about mixed classes is timing. Your pally does a judgement, DK does a disease, druid does a cat/bear attack, shammy does a shock. Now what happens is the shammy is done for 6 secs, the druid can do an attack almost right away, the DK is reallly ready and the pally is ready. Not bad, but things get messier from there. The DK doesn't need to do a disease for like 15 secs. The shammy may not exactly time with the druid's rage/energy. The pally wants to do more attacks (spells), but when should he time them? The simplest thing you might enjoy is doing 4-5 of the same class. Try 5 DKs. They have self heal and if you go blood you get group rune tap (30% or so heal). Pallies can put light on the mob, so all melee can heal and their timing is the same. It just makes things easier if you use the same classes. In theory you get more if you mix and match, but you really don't if you don't have seperate people operating them. You run into timing issues and then you do about 70% of what a same group does.

Things I thought about on melee. Facing (rogues, druids). If you aren't facing excatly like you want, you miss out on DPS. Range. If you don't have any ranged, you might get stuck in a spot where you need it (spawned mob that casts, runners and if you get frost nova'd/slowed). Silence. It doesn't happen often, but be ready to do damage without casting. Runners getting adds. And make sure you get a boxing addon (multiboxer or jamba) for following since you will break follow every pull.

Good luck. Hope that helps.