Firebats. Weren't they such a badass unit? Mow down grown forces like nothing. Since they are gone, the Terrans now have more trouble with all sorts of unit surrounds like ling surrounding. The hellions are NOT firebats on wheels, their splash is pathetic really and simply break like sticks.
Medics. Extremely vital unit in early stimpack strategies and are cheap counters to siege tanks and debuffs.
Goliaths. Though there is already a thor, I would like a ground anti air unit that doesn't cost buttloads and build time. Therefore, the factory units are easy prey to air units.
Spider mines. It is a shame that they are not in multiplayer, sneaky ganks would be countered.
Science vessel...defense matrix, irradiate, emp...yeah...
All other scraped units have nice replacements I believe.
REAAAVERRR. TOSS NEEDS A HEAVY SIEGE UNIT, colossus cant take out units fast enough. Also reaver drops and reaver bombs.
Arbiter. Recall may be one of the most awesome skills ever, sneaky arbiter behind someones base and then recall army inside. GG
Corsair. Disruption web is a very good skill, neutralize ground forces and tanks.
Lurkers. A zerg siege unit, stop-lurkers may be broken but that's what they're designed for and mass ground units wont stand up to them.
Scourge. Sick of void rays?
Defiler. Arguably the best spell casting unit in the history of starcraft.
We need these oldschool units back, though some of them now have nice replacements.