For me?
When Javik recognised the Crucible. I didn't want the Crucible to be some last ditch project by the Protheans to defeat the Reapers. When, on Mars, Liara said 'The Protheans came close to defeating the Reapers', I practically did a spit take. The point of ME1 - the point of the cycles within the plot - is that the Reapers have pulled this off hundreds, maybe thousands of times. The Prothean cycle was normal - the Reapers took the Citadel, isolated everyone, and took their time crushing each system in turn. If a normal cycle can come 'close to defeating the Reapers' the Reapers would have failed sometime before now.
(Heck, if a normal cycle even manages to take down two or more Sovereigns, then the Reapers should be losing numbers as time goes on.)
So from Mars, I was expecting there to be something more to the Crucible. When Javik recognised it... It confirmed Liara's story. The Protheans had indeed been working on a device that had the potential to defeat the Reapers. And that disappointed me.
In the end, it turned out that there was a bit more to the Crucible than it appeared at first glance, but it still relegated our cycle's victory to blind dumb luck. The Catalyst mentions that the Crucible designs have only been around for the last several cycles. That means the only reason we even had a chance was because of the Prothean Ilos sacrifice AND the fact that our cycle happened after the Crucible plans had been developed but before the Reapers had managed to successfully wipe the plans out.
If I'd been writing it, I would have had the entire chance at victory descend from the events set in motion by the Prothean Ilos team. A chance at victory from self-sacrifice works better than a chance at victory from blind luck.