I'm quite disappointed in the effects I've tried so far (except the water shader), because they are all advertised along with the rest of the features, and for the most part they are the same old well established effects I've been using for 10 years plus in various Adobe products: (glow, emboss, blur etc).
These effects run in real-time at high framerates. Image editing tools run effects offline so the performance doesn't matter. I think being able to do Photoshop-style effects in realtime with GPU acceleration is a pretty awesome feature.
The effects compositor is super complicated, as described in my two-part blog on it: part 1, part 2. This makes it difficult to change. It's also very difficult to do certain effects like blurs - and glows are actually a variant of a blur - due to the complexity of the effect compositor; the limitations of how GPU shader programs are written (providing only limited capabilities that are the small set of things GPUs are good at); and the difficulty in implementing them in a high-performance manner without hammering GPU memory bandwidth, which usually involves additional steps in the compositor to adjust the resolution during effect processing (which is another very complicated change to what is already by far the most complicated part of Construct). So, easier said than done! I think we actually provide a pretty good selection of effects built-in, there are more on the addon exchange, and the addon SDK also lets you write your own.