Good descriptive details about the mechanics of Maokai and narrated in a fervidly manner. I think this guide could really do with some depicts in order to interest the reader visually. I'd also envision that it would also enhance the guide to expand to other sections such as jungle path, skills, summoner spells etc.

Masteries

Honor Guard is a rather weak investment in that it really has no real notice-ability mathematically - Fox-Fire hits you for 350, you'll only get a measly 5.25 damage reduction from that via Honor Guard . Initiator is a much better replacement being a top notch talent as the tank you'll likely be initiating (bad pun?) which the extra movement speed boost will help get in range to land Twisted Advance same goes for any ganks being proceeded with. It helps defensively as well for fleeing from a dangerous situation.

Your very much likely better of putting that extra point in Indomitable rather then Siege Commander since it will help with your early jungle clearing and to assist keeping you healthy in your jungle in the chance that a duel or invasion may occur. You shouldn't really be wanting to die often and you'd need to die in order for the talent to cause any influence while Expanded Mind will duration wise be up longer and more importantly it keeps your mana healthily sustained to allow blue donations to your AP carry and enough mana to clear camps quickly and gank well.

Runes

Not having X runes isn't really an excuse to not include them even if you don't have these runes you should include these as you say "better runes" to give people an idea of options to weigh down. For all you know they may have these "better runes" but not the runes you have, putting themselves in a similar dilemma of your own. I'd include Greater Mark of Attack Speed for instance.

Items Build

I'd probably think its worth giving a mention to other defensive items as well such as: Banshee's Veil (against combo oriented AP champions like Brand, Sion etc), Frozen Heart (against auto-attack oriented AD champions like Vayne, Tryndamere etc).

Great guide xD Glad to see more Maokai players out there