I read the guide and downvoted because this guide doesn't take advantage of Singed's passive. I also noticed you changed your Runes as Mukhopper suggested.

On a side note, your win/loss ratio of 1/9 with Nidalee is terrible!

It's funny how you skipped the text, just looked at the items and downvoted.

As a tank and somebody who is going to want to get to the center of the fight, you are going to want a tenacity item, boots of swiftness really are not the best choice. You have a long ultimate buff and quints and ghost for movement speed. You will want the mitigation as well.

In terms of not building rod of ages, the reason it is so good is because of singed's passive, which should not be taken lightly, it gives a huge mana boost as well as 811 health and 80 ability power for only 3000 gold. Also, I have always thought frozen heart to be a better choice over sunfire for any tank, let alone singed with his passive. Rylais is a bad choice IMO because if you want people to be chasing you with your poison trail, you don't want to be slowing them to get out of it.

Also I don't think that mana is the best choice for glyphs, flat MR or scaling MR is a better option in almost any case, you will at least double your magical effective health pool with flat magic resist glyphs early game, whereas you get 25 health from those glyphs and 100 mana, which, yeah will get you another fling, but isn't worth it in my opinion. But I guess you need it for the shocking lack of mana or regen until your 3rd item.

My guess is more room for straight tank items rather than HP/AP.
Not that its right or wrong. That's just my guess.

if you don't build RoA for Singed you're doing it wrong.

it should be the first item

RoA is a core item for Singed. Why would you NOT want it?