The Leafs were expecting Steen and Colaiacovo to take the next step, and they didn't. Steen was struggling in 08-09 (20 GP, 2 G, 2 A, 4 PTS) and Carlo couldn't stay healthy. As both were first round picks, obviously in Toronto, this comes with a ton of pressure and expectations.
Steen had a very good rookie season (75 GP, 18 G, 27 A, 45 PTS in 05-06) but seemed to regress after that. As for Colaiacovo, he missed a ton of time due to injuries. Steen was 24 at the time of the trade and Colaiacovo was 26.
Stempniak was a good right hand shot that the Leafs needed, had a 27-goal season in 06-07, and put up 13 points in 14 games to start his 08-09. He was 25 years old.
Steen's poor offense continued in St. Louis in 08-09 (61 GP, 6 G, 22 A, 28 PTS) but Colaiacovo was able to stay healthy and was getting top-4 minutes (63 GP, 3 G, 26 A, 29 PTS). Steen, of course, finds his game in 2009-10, gets Hart Trophy votes in 2014 and wins the Cup in 2019. Colaiacovo stayed with the Blues until 2012, averaging 65 games a year, but seemed to be in a steady decline in his final year of his first stint. Colaiacovo did return to the Blues in 2014 for 25 games.
As for Stempniak, his numbers took a dive in Toronto in 08-09 (61 GP, 11 G, 20 A, 31 PTS) after his very hot start with St. Louis. After another mediocre season in 09-10 (62 GP, 14 G, 16 A, 30 PTS), the Leafs traded him away to the Coyotes for Matt Jones and 4th and 7th round picks, and Stempniak gets hot again with the Coyotes (18 GP, 14 G, 4 A, 18 PTS). Stempniak, a 5th round pick in 2003, would carve out a good NHL career for his draft position (911 GP, 203 G, 266 A, 469 PTS), but this trade turned into a complete disaster for the Leafs.
The immediate effects say Colaiacovo help stabilize the Blues defense as Brewer eventually had back surgery in December 2008 and left a big hole. The Blues did make the post-season in 2009. Long-term, obviously, Steen is by far the best player in the trade.