For me, MacPorts' main mistake was to go all in with the "storage is cheap, CPU is cheap" canard just as everyone was abandoning desktops for laptops. MacPorts decided to build its own copy of almost everything that the OS distribution provides, to avoid having to deal with version differences between OS X releases. That might have been a decent tradeoff on a Mac Pro, but when you have a laptop with a small HD (and, later, an even smaller SSD), it is a real issue to have to permanently waste several GB on duplicate copies of Perl, Python etc. just because you want to build the GIMP; to say nothing of the hours it would take to build them.
Homebrew's big selling point, for me, was that it could just use the libraries that already came with OS X.
FWIW, nowadays (for a few years?) macports has binary packages, so that is not as big an issue as it used to be, i.e. installing ImageMagick would take hours, now it just usually gets all of the packages and dependencies as binaries.
I think used space is not a huge problem either (I have 130 packages and it uses 1GB), the real issue is that it's anti-intuitive that macports does not remove inactive stuff and does not autoclean, so you end up with a lot of wasted space for stuff you had no interest in keeping.
What does this even mean? If it means "uncool" then, yeah, I get it.
If you are comfortable with Ruby, then it is easier to build packages for Homebrew, and since it isn't centrally managed like MacPorts, things generally get packaged quicker for Homebrew. On the other hand, MacPorts has a long track record and has generally been bombproof for me across 3 or 4 OS updates now.
As an existing MacPorts user, I looked at it from the standpoint of "what is this doing for me that MacPorts doesn't?" I didn't see anything that seemed compelling enough to switch.