Hmmm, I had a 1-night paid stay there on Oct 12 (Friday evening, so the hotel should have been pretty empty?) and while I did like the hotel overall (decor, room, location), I wouldn't say that I was impressed to the point where I would start comparing it to the Park Hyatts (especially the international Park Hyatts). Perhaps your experience was a bit tainted because it seems like you got an exceptional upgrade to the King Studio...but I think the norm at the GEH is to upgrade Diamonds 1 category up (i.e. from "City Single" to "Queen Double", or from "Queen Double" to "King Double"). In any case, I don't think the 1 category upgrade is all that significant, since the difference between the upgraded room and the originally booked room is only 2 square meters in size.
As for the hotel... I had the same feeling as you when I first walked in... The island was located slightly off-centered, with sofas next to it. If I'm not mistaken, this "lobby" that they use now is not the same as the "lobby/reception area" that is shown on the hotel's website. In any case, it is a rather refreshing concept, although I found it a bit confusing during peak periods when there are lots of customers around, as there was no line(s) to cue up.
Check-in was swift but other than the room upgrade, the staff did not voluntarily explain the other benefits of the Diamond program. For example, I had to ask about the breakfast benefit, as well as what the Diamond amenity was... For breakfast, I thought the GEH was very generous with this... They basically give you a credit, something like the £25.50 mentioned by Bondiboy for double occupancy. The credit can be applied through room service, or in my case, I opted for the "Terminus" restaurant. I thought the breakfast was very good quality at the "Terminus"...but I probably wouldn't shell out £25 for what we had if it's coming out of my own pockets. As for the Diamond amenity, the check-in staff had no idea what I was talking about and never got back to me. At check-out, I inquired again and the staff also was unaware of it, saying that the breakfast was the Diamond amenity. After checking with other staff, she finally came back and said that the Diamond amenity should have been the complimentary water in the fridge... Well, there was no sticker or card in or next to the fridge indicating that the bottled water was free...so, I ended up not getting the Diamond amenity. Hyatt GP has really got to do something about this Diamond amenity thingie. I think in the US, all of the Hyatt hotels pretty much know what they are doing. For international properties, the amenity varies greatly...sometimes nothing at all.
The room decor was quite modern and contemporary...the fridge, the safe, TV, furniture, etc., were all quite contemporary in style. The in-room safe didn't work in my room though...was locked before we even used it...didn't bother as it was a 1-night stay.
The bathroom in the King Double was just a standard bathroom... No separate shower and tub. No quick-draw tub. Don't quite remember the brand, but the toiletries were probably something that's just above average, nothing to write home about. Dental sets were only provided on demand.
As for the free Wi-Fi, free internet on TV, free minibar, free movies... I was not told of any of the above and quite frankly, I'm quite surprised that the hotel would offer so many things for free. I was told that if I wanted to use the internet, I could use the 2 computers next to the elevators on floor 1 (or 2?). Those computers were connected to a printer so one can print out boarding passes (no charge).
One last comment about the staff... They seem to be doing everything. They are receptionist/bell man/concierge all combined into one. When I got off the cab, there was no bell man in front of the entrance to help with the luggage. We had to go in to grab one of the receptionists. I was a bit confused about who to approach to help me with my luggage, or to help with a restaurant recommendation. I think this approach is fine as long as the staff can handle...but it's often difficult to do everything well all the time.
Again, don't get me wrong, we had a very nice stay at the hotel and enjoyed some of the more innovative approaches (e.g. I really liked to map/room key holder). The staff were also very nice. However, since it's still so new to the Hyatt family, it is still very much a work in progress before the staff get up to speed on all the fine points of the Hyatt program. I would not hesitate to stay there again during the weekends, when prices are more reasonable. During the weekdays, I think they charge something like £300~400 a night, which I would never pay for a 30 sq meter room.