Hmmmm, tough one:
-Deus Ex (2000): Didn't play until 2004.
-Assassin's Creed (2008): Owned for a couple of years but never played it until this month.
-Vampire Bloodlines (2004): Didn't play until late 2009
-Mass Effect (2007): Bought it in 2008 but didn't play more than half an hour until last summer
-The Witcher (2007): Didn't play until this year
-Freespace 2 (1999): Played it around 2006 I think
That said as for how many of those I genuinely regret waiting on, probably not that many because playing years after release means fully patched up, better hardware to play it on and better mods (SCP for FS2 yum yum

). Maybe Deus Ex as I knew some people who played it at the time so I could have joined in the conversation. Also I played Invisible War first and maybe would have appreciated the storyline more had I already played the original.
A fair proportion of the games I play are old (I think at least 30% over a year old and probably at least 10% 3+ years old) so fairly standard for me. Kinda weird though I do talk myself out of playing supposedly very good games. Examples e.g:
-Dead Space: Owned since 2009 but not a fan of horror genre so haven't played it yet.
-Battlefield 2 / 2142: Had for a while now but too scared to play online (probably would have made more sense to play before BF3 came out as presumably has a hardcore crowd now)
-Assorted Total War games: Worried about how in depth they are, gave Rome a try years ago and was a bit overwhelmed.
-Just Cause 2: Third Person view
-Civ4 & Civ5: Big fan of Civ3 so no idea why I haven't got round to these, worried bout them being a timesink mostly I think