Orient were trailing to Neal Bishop's goal until substitute Kevin Lisbie secured a 1-1 draw from the penalty spot.
But they also hit the crossbar through Jimmy Smith and had a Charlie MacDonald strike ruled out for offside.
Slade said: "I think we are a little bit disappointed to be honest. We feel as if there were three points for the taking and we've not taken them.
"The domination we've had is incredible. It's not just the possession but we've hit the crossbar and we've had one that was marginally offside ruled out.
"We worked hard to get back into the game second half but I think we were the side that was looking to win from start to finish.
"When this game was called off over Christmas you would have probably gone, if we had got a point that day, that's a good point at Notts County as they are one of the contenders.
"As it turns out, we were by far the better side on the day and deserved to leave here with all three points."