WinDirStat is a great open-source utility that shows you what's eating up your disk space. Give it a shot:
It seems like every time I run it on someone's machine, it finds some temporary/garbage files left behind by a program in an obscure directory totaling several gigabytes in size. The worst I saw was 86 GB of update files in a directory for Symantec Endpoint Protection. Yikes.
I, too, don't understand why Apple decided to replace the ExpressCard slot with an SD slot on a supposedly pro-level notebook.
The ExpressCard slot provided the only high-speed expansion option on Apple's notebooks. Maybe I'd understand this move if there was a docking station available that added other expansion options, but there isn't.
I do a lot of photography and often shoot gigabytes of raw photos in a single shoot with my dSLR (which uses CF, not SD). Yeah, the sort of work the MacBook Pro is supposed to be aimed at. Besides that, I also do a lot of work with large disk images for the IT work I do.
Doing such work on my aging MBP is a joy because I have an ExpressCard Serial ATA adapter that lets me use external hard drives without the limitations and overhead of USB, FireWire or ethernet. If I wanted, I could also use the card to connect to an external RAID enclosure at SATA II speeds.
What good are the performance increases with the CPU, memory, graphics, etc if the only expansion option that provided the quickest data transfer speeds is now gone? Disk i/o will be an even worse bottleneck for me on a new MBP than my old one. No thanks.
I was looking to upgrade my 2.5 year old MBP with a newer model, but I refuse to do so until Apple brings back an ExpressCard slot or something better.
If all else fails, lower your standards.