I think you're a bit too creature-heavy and spell-light. It seems to me that you have a lot of spells and abilities that utilize counters very well, but not too many ways of generating them. You should probably try to cut it down to about 18 creatures and replace the difference with Heroic-triggering spells. Gyre Sage and Horizon Chimera seem somewhat out of place in the deck IMO.

I'd also recommend narrowing down exactly what you want this deck to do. You have a lot of 2 and 3-ofs in the deck, and taking out some of those while upgrading others to 4-ofs would really increase the consistency of this deck.

As for spells, I'd recommend finishing your playset of Solidarity of Heroes. That spell is just ridiculous with Heroic units. Bioshift also seems amazing (I've never personally used it though) with Heroic units so that should be a 4-of as well.

As for other spells for your deck I think Nature's Panoply would work well with Battlefield Thaumaturge's ability. If you wanted to go more counter-heavy Hardened Scales would be fantastic as well.

Protean Hydra is always a fun simic card, especially with Bioshift around. I'd also suggest Thrummingbird and maybe Aspect of Hydra as a heroic enabler.

unfortunately Thrummingbird doesn't target, though it is good for just pumping stuff up.

I didn't mean that Thrummingbird was a heroic enabler, the suggestion was just Thrummingbird and then Aspect of Hydra as the enabler...

ah, my bad. the wording seemed to imply it and i had to reread the proliferate reminder text just to double check. if going modern, Strength of the Tajuru or Blessings of Nature might be picks to proc heroic. but i think this deck might have been abandoned, not a lot of commentary from the creator as far as i can tell.

Just started playing magic a few weeks ago, as such my comments my not be very helpful. However, I do like that you've made a budget modern deck from a standard deck based around an interesting mechanic. In fact this may be something I pursue when/if I get into modern.

Some quick questions though. How much success have you had playing this deck? What's been it's key weaknesses?

Nice deck +1! With the amount of counters, maybe think about throwing in 2 Tuskguard Captain and/or similar cards or maybe sideboarding in to fight off weenie decks? Also, what do you think about a hexproof sink for counters if you're facing a deck that's good at fighting off counter's based decks like blink/flicker decks?

One thing to think about is the new Elspeth vs. Kiora duel deck set that has recently been released. There are quite a few good Simic cards in there, such as Kiora's Follower. However, that deck has a "ramp into big creatures" theme as opposed to standard Simic's "+1/+1 counters." A good Simic +1/+1 counter creature is Master Biomancer.