Being fortunate enough to live in Melbourne where quite a few of the bleeding-edge third wave/specialty cafes are located (St Ali, Sensory Lab, Proud Mary, Ona Coffee, etc), I would like to think I did get to taste some of the well-executed light roasts.
In my experience, they can be very different from what I remember tasting back when the third wave coffee movement first started (think double ristretto made with lighter roasts).
Here, the trend seem to be shifting towards serving coffee with more dilution and with lighter crema (often just 2mm layer of crema), 'thin' in comparison to the typical Italian espresso. It can be very balanced (when well executed) and the focus is more on the delicate flavors - there is noticeable amount of acidity/clarity but certainly not what I would call OJ (the acceptable level of acidity can be subjective depending on the individual too).
Think of it as a concentrated form of filter coffee. It won't be your thick syrupy emulsion that many of us have grown accustomed to. People who can only enjoy filtered coffee in the past and disliked traditional espresso will like it (perhaps those who have a more sensitive palate and are overwhelmed by the 'strong' espresso sensory experience). People who enjoyed the syrupy espresso they've known all their life will probably balk and be shocked at the 'thin' (relative) swill.
If you come in expecting a flavor bomb experience that a traditional espresso provides - flavors that just dominate every aspect of your sensory and flavors that evolves in your mouth minutes after sipping, you will likely be disappointed with the lighter roasts. My observation is, the flavors from lighter roast espresso can be complex and have tons of clarity, but they tend to be more front-loaded and short-lived - meaning you taste most of the complex/delicate 'origin' flavor when sipping, but it will disappear soon after (relatively of course). It's just not the tongue coating experience that a traditional espresso can provide.
To me, they are just different style of drinks and I can enjoy both (and know some who prefer one style while completely despise another). If you're used to conventional espresso and come in with the same expectation, it will likely be a disappointing experience.
To make those lighter balanced shots, having flow/pressure profiling and going out of the typical extraction parameters are often required. If you just hit those light roasted coffee (provided roasted properly) with 25-30s extraction, 1:1.5-2.0 brew ratio, 9 bar flat, more often than not it'll come out as a niche drink that only a few will come to enjoy. The key to these lighter roasts seems to be dilution - if you pull more volume or more water through, then it's more likely to be non-offensive and allow those flavor to become distinguishable and enjoyable. But also due to that dilution, it is perhaps lacking the experience traditional espresso drinkers are expecting.