uh oh
Edit: on reflection it might be a genuine question so...
1.6 benefits:
gameplay (feel of the game, the way it plays out, personal opinion)
user maps (some of which still aren't matched in source really imo),
community (back then, nothing to do with the game)
bad things:
bugs and exploits,
balance, AWP was silly as was the desert eagle. Some of the maps were pretty poorly designed
skins and sounds have aged badly
Source good bits:
source graphics
modding capabilities (in our last lan i made skins of some of the players was a piece of cake to make very good ones with the tools provided by valve)
more options in server configs and great new features like the HTTP download (now with gzip capability)
half life tv, record videos with little to no hit in performance then playback and record using your favourite program afterwards.
Some great user made mods, e.g. zombie source, hidden
some bad bits:
worse gameplay (personal preference)
maps made unecessarily complex (extra pointless items in the world just creating clutter)