| | Re: would appreciate it if
Lexical verbs divide into two categories : transitive and non-transitive. (That's a huge simplification, but it will do for this case.)
Transitive verbs are always followed by an object - for example : I keep rabbits; John hugged Mary;We made a cake - you can't just say *I keep, *John hugged or *We made
Intransitive verbs, on the other hand, don't need an object : It rained; David laughed; Mary coughed.
Appreciate is a transitive verb - it must have an object : I appreciated his offer. You can't just say I appreciated - you must appreciate something.
Subordinate clauses starting with "if" can't act as a grammatical object, so in your sentence we need to find an "empty" object. Whenever an "empty" subject or object is needed , "it" is always used (think about It rained. It doesn't make sense to ask What rained? "It" is only in the sentence to provide the grammatical subject which the verb must have - it's "empty" of real meaning).
Or another way to think about it is to consider the sentences :
He replied immediately. I really appreciated it.
Here "it" clearly refers back to "the fact that he replied immediately". Now change that into the sentence you are asking about :
I should appreciate it if you would reply immediately.
It's really just the same thing.