GCP is discoverable and follows the principle of least surprise quite consistently, and they show REST/CLI references for everything you're doing in the web UI too.
I mean imagine you're a new user and ask yourself "what does route53 do?". Then ask yourself "what does cloud DNS do?"
The AWS UI doesn't handle multiple subscription, it's somewhat annoying when you have multiple accounts (let say dev and prod ones). With Azure, you can federate several subscriptions.
Having no consolidated view for all regions is also somewhat annoying, as it is to search a specific resource since these can only be searched in their corresponding panel (ec2 for instances and ELB, RDS panel for DB, IAM for certificates, etc). In contrast, the Azure UI allows you to search accross all resource types in one search box.
There are other issues where I don't have a point of comparaison, but:
If you have a lot of certificates/domains, selecting the correct certificate in an ELB or a cloudfront distribution serving HTTPS is a pain. Same with selecting security groups for an RDS instance or an ELB.
The AWS UI is also painfully slow if you have a big account. The RDS panel used to timeout past 600 or 700 instances, it's a little better now (yay pagination \o/), but still quite slow (10 to 20 seconds to search for a specific instance). The EBS snapshot UI is not usable past 100000 snapshots.
I do agree that as long the API is working, the UI is not critical (don't get me started on API throttling, which are really painfull to track down and handle). It would be nice however if it worked reasonnably well, a UI is still quite efficient when you have to explore and understand what is going on.
It is annoying dealing with multiple AWS accounts.
To mitigate this, you can supposedly use IAM to switch your current IAM User role to a role owned by another account.
The UI is great for learning about services, but automating via CLI (or related services) is the ideal goal.