I saw this post on my pc, but as if to prove a point I replied on my iPhone...
The only annoying thing I can think of is that the iPhone doesn't deal well with text boxes. If you reply to a long post, well you just can't there are no scroll bar and no arrow keys, so reviewing what you've written is sometimes difficult...
If I'm replying in detail to a long post, I generally copy the text and use a word editor to write my reply, then I copy it back to the forum reply box once I'm done.