How to limit the free user was a though decision for us. You can increase your document space without buying premium: for every user you invite both you and your referral get 5 additional docs. Also, a free user can download and store his docs locally and enjoy Quabel for free for an infinite time.
Great work. You may want to consider switching the "Add Task"-div with the "Daily/Monthly/Future"-div. So that the "Add-Task"-div is aligned in the lower left hand corner. That would be way more intuitive for me. Another great feature would be rearranging the existing tasks.
I guess with HN it's a good idea to stick with the old design. A new (fancy) design would only attract more mainstream users which is not in the interest of HN in the first place.
You should consider buying an mb air instead of an mb pro. The ssd makes it unbelievable fast and paired with an extra display it's a power house for development. the mb air is just a solid piece of hardware. I have no regrets buying one so far.
Skeumorphism design may help me to adapt to the piece of software the first couple of seconds. However, after that it distracts me the rest of the time I am using the software.