I am looking forward to W10 for my phone. I do hope that one of the features coming is the ability for the start screen to be rotate to landscape. There's a number of things I do in landscape mode and also have a case with a kickstand to set my phone down without holding it. Having to jump to the Start Screen and it being locked sideways isn't fun.
In regards to the worry of system specs, I have a 930 so I know I have nothing to worry about, but I doubt anybody else has anything to worry about either.
Think about it. Microsoft just released a number of low end models in the last few months. It makes no sense that they would release a bunch of 400/500/600 series phones for them to only become obsolete in just a couple of months when W10 is released.
Anybody with a brain at Microsoft would know what kind of backlash this would cause with those who just bought one of these phones.... A backlash they don't need right now especially when the cost would be the gains they made in the low end market.
And still, system requirement specs still need to be released for a number of reasons. For starters, other OEM's out there need to know what their phones will need to have in order to run W10.... And there have been a lot of OEM's out there who have released a lot of really poor and low end cheap phones for third world markets that run WP8.1.
In regards to Lumias, the worst case scenario is that some lower end models will have limited or restricted functions (ie: Only the Lumia 930 & 1520 get Hey Cortana in the latest 8.1 update)
The comments relating to how they will try and get W10 to as many Lumias as possible, that is more towards the older Lumias out there, not the lower end Lumias. Lumias with older hardware like say like the 810 or similar generation, which run Windows Phone 8/8.1 may not make the cut for W10, while the X20 series phones may make the cut.