Developers have some things in common...
Developers understand english.
Do not limit availability of a developer laptop from countries based on the language support in the OS. Most will want their native key layouts, but offer US english keyboard as an option in all markets.
Developers use specialised and wildly varying toolsets and they know how to install the tools they need.
Preloading something that can be installed with a one line "sudo apt-get install ...." command is utterly pointless. This is what we have software reposities for. One mans essential dev tool is another mans bloat.
If you are thinking of providing special tools only available with this line of laptops: think again. Any developer using a free OS will understand the concept of lock-in, and will know to avoid it.
Developers need to see lots of text on screen. -> High resolution matte display.
Developers write lots of text. -> Need a really, really, really good keyboard. As many have mentioned, a full set of [cursor,home,end,pgup,pgdown] keys is also required.
Make hardware that will work with Ubuntu without special treatment or workarounds. The best way to accomplish that is for the drivers to be in the Ubuntu software repository or better yet; in the mainline linux kernel. When people talk about open source drivers, it is not just open source zealotry, it is the pragmatically best way to get reliable drivers that are co-maintained with the kernel. Closed source drivers always tend to be one version behind the rest of the software stack.
When closed source drivers cause problems, the problems are much harder to troubleshoot. This only becomes more annoying to people who would otherwise know how to troubleshoot software.
Everyone needs a good touchpad and connectivity.
Everyone needs good battery life in a laptop.
Very few need spinning optical media drives in a laptop.
General laptop design tip: don't put ports so that the attached cables are in the way of where a mouse would be used.
Do not make surfaces that are meant to be touched by human hands in a material that look smeared with grease the instance someone touches it.
Many (though not all) will use virtual machines in one form or another, they need a powerfull CPU with hardware virtualisation support and multiple gigabytes of RAM.
If you specifically want to make a laptop for ubuntu, the bios should have linux friendly default settings.
UEFI secure boot is a big hairy problem we could do without. Please at least look at Coreboot+grub as an alternative.