I've never made a BUG deck before so I figured I'd give it a try.
Abrupt Decay: Cheap removal that can't be countered. Awesome for getting rid of that pesky Umezawa's Jitte or that Liliana of the Veil who keeps harassing your shit.
Ashiok, Nightmare Weaver: Mainly in here to help me steal creatures, as this deck isn't very creature heavy.
Counterspell: Because any blue deck isn't complete without some counterspells.
Creakwood Liege: Can create some much needed 3/3 tokens each turn.
Delver of Secrets
: Turn 1 bomb to start applying pressure.
Hero's Downfall: Creature and planeswalker removal to keep the board clean.
Hymn to Tourach: My favorite card in the deck. Absolutely shreds opponents hands early on.
Inquisition of Kozilek: Great early hand disruption.
Kiora, the Crashing Wave: Can protect me against my opponent's big hitter, draw me cards, and then create an emblem that basically ends the game.
Liliana of the Veil: Discard and sacrifice, anyone?
Mana Leak: Something, something, blue decks and counterspells...
Preordain: Great digging card, I actually prefer it to Brainstorm and Ponder as it allows me put cards I don't want on the bottom.
Remand: Counter/draw.
Serum Visions: A shittier version of preordain, but still useful.
Tarmogoyf: THE GOYF!!!
Thoughtseize: Yummy hand disruption. Let me just take that piece of your combo away...
Vampire Nighthawk: A great blocker, and can help me gain back the life I'm losing to shock-lands, fetch-lands, and thoughtseizes.