"Secret of My Excess" is a more humour-focused episode, with a lot of its strengths deriving from the jokes, action, and fast pace. Indeed, it's an episode packed with jokes, and remarkably few miss the mark. On top of that, the episode provides some good character development for Spike, just to push itself up to "great" status.
The episode isn't quite as gag-stuffed as, say, "Feeling Pinkie Keen", but it's still a particularly humour-oriented episode, with a great deal of the episode comprising jokes. This proves effective because, as said, the jokes mostly land, resulting in what is a very funny episode. Humour is mostly standard MLP fare, but it's still far too well executed to be stale. The jokes often come in rapid succession, but are paced very neatly so that they keep the plot moving forward.
There's also a fair amount of action in the episode. It's pretty quickly paced, but unlike "The Mysterious Mare-Do-Well", scenes flow into each other, and each one serves a purpose. After the episode is set up, it lets loose with the action, as Spike rampages through town, and the mane six conspire to stop him. Action in this episode is well-directed and exciting, without a great deal in the way of fight scenes but boasting an entertaining amount of minor city carnage and general rampage.
Spike gets some nice character development here, managing to overcome his impulses through the memory of an earlier action. It's a bit questionable if he really evolves much here, especially as his greed is caused by natural impulses, but he does show greater maturity than he has previously, which at the very least implies some strong character growth. Also interesting is the clear way that Spike's descent into greed goes, as he transitions smoothly from just mentioning his birthday in passing to attempting to use it to get gifts.
"Secret of My Excess" is a very entertaining episode, with great humour, great characterization, and overall great writing. Its fast pace and fun action serve to make the episode exciting, and altogether it's just a fun watch through and through.