In my opinion you have way too many unique cards.
If you find the cards in your deck that work the best towards your goal, which I'm assuming is "beat face with big creatures", and drop the rest, then you'll have a deck that will perform much more consistently.
I'm not going to comment on your creatures, except from the fact that you should have 4x of the creatures you feel like you most often want on the field. I would probably also drop Somberwald Sage and swap him for Avacyn's Pilgrim . By turn 3, you should have other things than ramp that you want to play.
With a CMC this high, even with your mana dorks, I would run at least 23 if not 24 lands. I would also drop Grove of the Guardian to a 3-of.
I would swap Selesnya Guildgate for Temple Garden just so your Arbor Elf has another target to untap. I would also say take out the Vitu-Ghazi Guildmage for an extra copy of Trostani, Selesnya's Voice , just because the guild mage is to expensive for his ability compared to Trostani, plus adding another copy of Trostani, increases your chances of drawling her.
The reason why I'm reluctant towards Somberwald Sage is because; a) He's in a cramped spot on your mana curve with 3CMC, and b) he can't ramp for you until T3.
I think there's quite a few obvious changes, however I don't know anything about your budget or if this is just cards you have already. If you want, I could post a more in-depth list of changes that I feel would benefit you?
For example you could definitely include Restoration Angel cut Roaring Primadox (I'm 100% sure that he can be replaced with better cards such as Advent of the Wurm ). Also cut Vitu-Ghazi Guildmage ; as MagicDC pointed out, Trostani is just better.
Ah, also, I would AT LEAST sideboard, if not mainboard, Oblivion Ring . This will definitely help you out.
If budget isn't an issue, then you could go for Garruk, Primal Hunter as a way to refuel your hand.
I'll take an extensive look at it tomorrow, as I do not have the time tonight, fear not though - I will be back :)
So I've done my best to take a look at the deck. I feel this would run a lot tighter, and more consistently than your current deck. However feel free to make a list like this and playtest it a bit before you make any changes.
-1 Wayfaring Temple I'm torn with this one. If you feel he puts in a lot of work then you can keep him, however I think there are better cards out there. Id minus 1 him to a 3x.
Sigarda, Host of Herons Excellent card and a good facebeater. She excels against many of the sacrifice effects out there, but might be better in the sideboard. Depends on your local metagame really.
About your mana-base; if you don't want to shell out for Temple Garden s and Sunpetal Grove s, then I completely understand. That being said; IF you have the cards or wish to buy them, then I would definitely add 4 of each. If you get the Temple Garden s then keeping the Arbor Elf becomes much more appealing.
+1 Armada Wurm a good card that provides healthy bodies for your population strategy.
+3 Thragtusk is debatably the best creature in Standard, running less than 4-of seems a bit silly to me (even though he is HEAVILY overused).
+1 Trostani, Selesnya's Voice I would up to 3x, she is a win-con for you, consistently churning out big creatures whilst keeping you at good health. You definitely want to draw her.
+2 Gavony Township can help apply pressure if youre out of cards lategame. Provides at least 1/4th of the boost in creature power compared to Grove of the Guardian, and will most of the times be outright better, as you'll have two chump blockers.
+3 Selesnya Charm , so much utility. Serves as a pump spell, removal or in worst cases a blocker.
+2 Rootborn Defenses Populates and protects against sweepers.
+2 Oblivion Ring This removal is too good to ignore.
+2 Advent of the Wurm a 5/5 Trampler for 4 mana at instant speed, cant ignore this one.
+2 Restoration Angel is just great. Protects against a killspell, is a blocker with a decent behind, and also flickers Thraggy or Armada.
+1 Land; you have to move this around by yourself.
-2 Champion of Lambholt I dont think this is useful for a deck like this. Cut.
-1 Craterhoof Behemoth A great finisher, but with the amount of fatties you have in this deck, I feel this is just a Win more card. Cut
-2 Frontline Medic Also an amazing board protecter card, but many times I think youd be more happy drawing a different card. Maybe a sideboard card.
-1 Roaring Primadox 4 mana for a 4/4 is meh. I dont feel his ability does anything good for this deck.
-1 Wolfir Silverheart I feel theres better cards for THIS deck out there.
-3 Grove of the Guardian a very situational card. 4 of is too many, however if drawn late game can turn two of your mana dorks into an 8/8 each turn. Keep a singleton.
-2 Bonds of Faith better removal included
-2 Growing Ranks too suspectible to removal itself and is slow.
-2 Parallel Lives too much of a win-more card.
-3 Call of the Conclave New 2-drops that are better in my opinion.
-2 Selesnya Keyrune Better ramp added.
You may also want to consider Seance as it works well with the populate mechanic.
Stone_Munkee: That's true, but a single 8/8 is much more susceptible to removal than an army of creatures that grows by +1/+1 each turn.
This deck's strategy is "turn-em-sideways", and thus having two untapped creautres just doesn't seem right to me.
But it's a matter of taste.