+ Aether Orbiter is not that useful (since it also prevents you from casting your own control spells and aura enchantments etc), I suggest making him only 1U.
+ Siege paladin looks a tad strong if he has vigilance, even with the attack every turn limitation. Maybe make him a 2/2 so that he won't plow through most early game non-defenders?
+ Decay Imp looks somewhat too strong in certain decks. Need a higher cost on the tap ability, and/or make it only affect 1 target.
+ Sludge Elemental is too strong. 1B for the ability to put -1/-1 counter on everything is pretty cheap. The 7 toughness also makes sure it'll be able to do that for a long time. It'd say make the CMC higher, and make the untap cost higher and/or nerf the P/T.
+ Summoning Song is kind of too strong, since green can mana ramp a lot. I suggest taking off the draw and make it monogreen.
+ Terra Beacon as is is too strong as sideboard material, since you get to destroy an object that is both a land and a threat for no cost. That ability should at least also require you to sac Terra Beacon as a cost.
+ Titan Horn is almost identical to but weaker than Primeval Titan, so you can afford to make it stronger. Unless you want a Primeval Titan that doesn't wreck Commander.
+ Volcanic Smog's flashback cost is too high, maybe change it to something like "Flashback 3BR. You may sacrifice a Moutain and a Swamp instead of paying the Flashback cost."
+ Vitality Split is a tad too strong when used in combination with things like scavenge and creatures with X in mana cost that comes in with X +1/+1 counters. Maybe make the CMC somewhere like 5-6?