I don't have a single deal-breaker. Every edition (even my beloved Moldvay and 4e) have bits I don't like but am willing to work around.
THAC0? Okay, I'll do the math.
Vancian spellcasting? I can make it work.
No tieflings, drow, warforged or [X]? I love creating races.
Alignment? Ignore as...