The two major ships of iCarly, Seddie and Creddie, are the ideal ships for appealing to both genders and for everyone from the good kids to the bullies to the geeks. I think this is one of the reasons that iCarly ship wars are so implacable. We shall look at the likely appeals of each ship in turn based on my analysis of the postings on the iCarly wikia, the fan boards, and the fanfic. This analysis may not reflect your shipping, but it does seem to reflect the majority.
Creddie's is likely to appeal to a female who overlooked a boy who crushed on her because either he didn't have base appeal or she didn't realise his good points until he moved on.
Seddie's appeal to males is similar. The difference is that the boy overlooked a female for either sound or unsound reasons - but he eventually discovered her appeal and good points until she found someone else.
Creddie appeals to males who crushed on an idealised female, but he was always overlooked for others he thought were not worthy of her.
Seddie appeals to females who were always in the shadow of the more appealing friend and could never get the attention of the males around them.
Essentially, the appeal of these ships for any circumstance basically comes down to one point: redemption. Carly stops overlooking Freddie and discovers the bad boys are bad to you too. Freddie opens his mind to possibilities beyond his childhood crush and realises that the bad girl is not bad at all and has done a lot of good for him. Freddie's persistence finally pays off and he is no longer the unappealing geek.. For once, the guy chooses Sam the sidekick and shows that she has put her jaded past behind her.
These appeals are very strong. Oddly, when a lot of these romances happen in real life, they are often someone setting for the other; i.e., that person is "good enough" or "can't believe s/he got that lucky." The hard numbers are in the final part.
Next: the hard numbers.