To keep this to the point I'm just going to talk about three comparisons but feel free to comment if you think of any more.
1. The obvious one - they both have magic that makes the story possible. Harry potter magic is controlled by a wand and Skulduggery Pleasant magic isn't (with the notable exceptions of Necromancy and Inhabiting) but they both use magic in some form or another.
2. Lord Vile (SP) and Lord Voldemort (HP). So, so many comparisons there.
- Their names. They both fancy themselves as royal or risen above the rest of the world. Lord Vile has a pretty clear meaning, although in contrast to many other ultra-villains he seems to recognise his evil by calling himself Vile. Usually a villain thinks that what they are doing is the right choice for their purpose, whether it's world destruction or whatever. I know that, obviously, the author chooses the names of the characters but why would Voldemort call himself Flight-From-Death (in French). Surely some of the Death Eaters can speak French and that isn't very good for Voldemort's image of the big, scary, undefeatable bad guy. Yes, it lets us know his whole purpose in life without him just saying it (show, not tell) but you'd think he'd be called something stronger, something that strikes fear into the hearts of his opponents.Don't get me wrong - I think he has a very interesting backstory, but if Flight-From is in his name it makes him seem like a coward. But maybe that was the message - Bullies are actually cowards underneath.
- Their background and back story. SPOILER ALERT. Lord Vile is who Skulduggery became when his family were killed and he was consumed by rage after coming back from the dead. Tom Marvolo Riddle became Lord Voldemort, a name that he said would soon strike fear into the hearts of all who heard it. Tom Marvolo Riddle is an anagram of 'I Am Lord Voldemort.' He hated his family and killed them all, so they both ended up with no family. Their lives are fuelled by hate and they feel no love for anyone. Fortunately for Skulduggery, with great power of will he is able to return to himself but Lord Voldemort is too far gone, even though it appears that Lord Vile has killed many more than Lord Vile has - 'whole battlefields', 'eventually I lost count. Everyone did.'
- Lord Vile is probably one of the only people who could take on Lord Voldemort and be assured of winning or at least living to tell the tale, without Harry's unbelievable luck. While Darquesse might be killed by the Avada Kedavra curse if she hadn't encountered it before so she didn't know what a spell was (although if he used Sectumsempra or something like that she'd stay alive because of her awesome healing powers), it would be difficult for a spell to get past Lord vile's 'impervious' armour. And without something like that, while Voldemort was trying to melt his armour or something, lord Vile could easily take Voldemort's head with a shadow. Although there is the matter of the Horcruxes. Still, Skulduggery is a detective - he can track them down. This is with Harry out of the picture. Oh my God, imagine a HP/SP crossover...
3. 3 ___________ should do it.
- In Harry Potter this is significant because it is the only way they can save Sirius. Dumbledore says 'Three turns should do it', referring to the Time-Turner.
- In Skulduggery Pleasant Solomon Wreath says 'Three billion should do it' when explaining about the passage, referring to the fact that in theory, for the Passage to work, three billion people would have to be killed which completely turns Valkyrie off being the Death Bringer.