Thresholds and expectation thresholds
  Consider a random graph G in G(n,p) and the graph property: G contains a copy of a specific graph H. (Note: H depends on n; a motivating example: H is a Hamiltonian cycle.) Let q be the minimal value for which the expected number of copies of H' in G is at least 1/2 for every subgraph H' of H. Let p be the value for which the probability that G contains a copy of H is 1/2. Conjecture: p/q = O(log n). Related conjectures for general Boolean functions, and a possible connection with discrete isoperimetry are discussed. 
Jeff Kahn and Gil Kalai