Ok, i'm downvoting and this is why.

First of all, ignite isn't a great summoner for him. Ignite is typically used for bursty champs that have to wait for abilities to come off of cd in order to do a skill rotation against. Karthus has no rotation and super-low cooldowns. Therefore ignite is not optimal. You even acknowledge that you're iffy about it. If that was the case why'd you put it in over teleport or exhaust. I think most summoners are subject to opinion and playstyle, but some are illogical, this is one. I personally run ghost/heal for early game survivability. Ghost also helps you keep runners in your defile.

Quints for flat HP. these would be ok if he was a health tank or jungler like mundo. This is not the case however and as such I'd advise you to switch these to flat AP so you can do some damage early on. Some other advice would be to go with AP per level instead of the mana regen runes you have (but thats just my opinion.)

Starting a blue crystal. It is my personal opinion that to be well balanced early on you need to have SOME mobility. The problem with this starting set is that you can't get away or dodge as well and dont have the additional hp pot to deal with the additional damage you'll sustain.

Try adding some more bbcoding, I'd suggest jhoijhoi's guide to making a guide.

I'll upvote when the above issues are resolved. Just PM me and i'll re-review.

Very good guide. The information is very detailed (much more than I thought it'd be) and the writing is very lucid and accurate.

A few notes -

1. You should move your Tear before your catalyst to get the tear stacks earlier.
2. You should get Summoner's Wrath for your Ignite bonus.
3. The guide has several 'wall of texts', especially in the 'playing as karthus' section. If possible, you might want to use different BBC coding, add in colors, different pictures, and so on to split it up. Use of borders such as '

---

', or some of the custom made borders would be extremely useful.
4. A late DFG would lack effectiveness as you'd usually die if you get too close, and although it isn't necessarily a bad thing with him, you won't be able to use it the best of your advantage (lack of burst, lots of magic resistance, and so on). Rather, a WotA, Banshee's Veil, or the best choice - Zhonya's, would be ideal.
5. Personally, I'd rather get AP quints then HP quints, just because of how his Q is spammable and does 2x damage to single targets, the AP really is amplified and its damage is really shown.

Good guide, +1 from me. With a bit of editing here and there, this guide can surely go up.

Don't forget to +rep me