What I have noticed more than anything is the change in routine -- there is definitely more of a sense of the day in / day out mentality, in my experience.  For example, picture life without your summer vacation.  Every week is a work week, unless of course you have pre-booked vacation or a leave of absence.  For a creature of habit such as myself the 'daily grind routine' can set in moreso than with other people, almost developing into a lull.  It's all relative to one's situation of course, as different careers will dictate.  
With a mortgage, your options for entertainment become much more limited.  We rent DVDs instead of going to the movie$, order in rather than dine out, and pick our 'splurge' items more carefully.  For me it might be computer hardware or my green fees, for her it will be shoes / clothes, etc.  In time we will have more financial freedom to have fun and purchase the inevitable toys.  I'm seeing the basement finished with a bar and a 60" plasma on the wall.  Hell I can already taste the Jagr-bombs going down after 18 holes...
I partied my ASS off all through my university days and will always miss those times.  So many fun houses, so many stories.  I met my wife at one such party.  Some of the friendships that came to fruition throughout school are still alive and stronger than ever today which is the best part, others are completely fizzled out because some people really do go their seperate ways or just live too damn far away.
I don't know... sometimes I'm painfully aware of that partying kid still alive inside of me... life after school isn't always gloom & doom.  Personally I like to think of the years before people have kids (if they decide to do so of course) as some choice living time as well.  There is still quite a bit of freedom apparent, but the spectre of CHILDREN hangs on the horizon - negatively stigmatized by all of the twenty-something couples around you who will fill your ears with horror stories at the loss of free will and free time.  
There is something to be said of the way society and culture impose "stages" on our lives... and how sometimes we can't help but feel trapped in them.  That's a negative view of course since we're all free to live any way we want.
Ah, life is good.  Thanks for the opportunity to babble.