I think the station has some issues (bad signage being an important one), but firstly the complex is still not finished yet, and being an essentially brand new station there will be certain bits which won't work correctly yet. When the new Kings Cross complex opens next door I think it will be a fantastic transport interchange.
I bet Euston didn't work perfectly the moment it was opened (It still doesn't if you ask me)
Plus, you can't really blame the new station for the walk from the tube tunnels as the tube tunnels haven't moved from where they were built 50/100years ago! - you can't really expect someone to spend literally billions of pounds moving the whole tube station just to save you two minutes walk!
Personally, I do think the East Midlands platforms are a bit isolated and poorly equipped, but at the time the station was rebuilt, there were no plans to electrify the Midland mainline, and it's quite clear that the design was for a modern electric station, hence the midland lines being stuck outside the central arch. Whether that was for the benefit of keeping those in the champagne bar from choking on diesel fumes or not is a matter for debate....