Expert mode removes compatibility checks. This shouldn't be the case - as soon as I select an item, it should cull the results and show me only the intersection of items that are now compatible with my chosen piece of hardware.
The lack of an ability to filter. Maybe I want to splurge more on a graphics card, but I need one that has at least 4 mini-display port outputs to drive my monitors. Right now there doesn't seem to be a way to say "show me only gpus that are nvidia, above $500, and can drive at least 4 monitors".
One of the biggest things for me: lack of reviews. You have a pangoly score. That's great and all, but you're a brand I don't (yet) trust. If I'm buying a component that costs multiple hundreds of dollars, I want more than a single score from a company I haven't heard of. I want to know what the reliability is like, and I want to hear it from people who have had hands on experience with it. I want to know how loud the fans are, what temperature it runs at passively and actively, etc.
Visually your site is really nice, but your expert mode leaves a lot to be desired, and I think you have a ways to go before you catch up with the competition. If I were you I would try and find your niche - either target users who are switching over from console gaming, and want to build their first computer, or build out a bunch of more expert features and target regular hobbyists.