Valves intentions for Source were purely to polish and shine the original game, if it ain't broke don't fix it.
Counter Strike was originally a mod made by two people for Half-Life Classic. It quickly became one of the most popular online shooters of its time and valve saw this as an opportunity, eventually hiring the two creators of the mod. These two modders then went on to help valve design the current source engine (hence the name Counter Strike "Source") now used in all of Valve's games such as Team Fortress 2, Half Life 2, Portal and of course, Counter Strike. This engine is now widely renowned as one of the best engines in video gaming, and has also been used in totally different genres of video games such as Vindictus, Nexon's new MMO, showing its versatility.
To any fans of the previous Counter Strike 1.6, it's fairly easy to convince them to buy Counter Strike Source, as it's everything they loved about the original with beautiful visuals, much better modding options and gameplay that's still as fast and smooth as ever. But how about recommending it to new players? For the FNGs, Counter Strike Source's problem is that it's way too focused on catering for the experienced players. The concept may be fairly simple to get your head around, Terrorists versus Counter Terrorist forces, one primary weapon along with one sidearm per round which you buy along with grenades and the latter before the round commences, you sit out the rest of the round when you die, and you win by planting or defusing the bomb, protecting or rescuing hostages, or killing every other member of the opposing team. But it's a b**ch to master. Valve originally planned on including an AI bot mode for new, inexperienced players, but to my knowledge I don't think one was ever added, however some clever modders have most likely created a server for such a mode by now. In the meantime, players are left to learn the new maps in hazardous online environments, filled to the brim with pro's that know how to kill you before you've even had time to buy the right gun, or find the best route to point A or B. In fact, they probably won't even know what point A or B looks like for some time! Then there's the advanced stuff, strategies, vantage points, radio commands, how to handle the weapon. All of which everyone knows already, so where's the fun in being constantly shot in the head by an AWP or an AK47?
Gameplay improvements are small, but worth noticing. You can now smash crates, windows and computer screens amongst various other junk you'll find littered around the various maps. The water now has audio effects, so it's easier for your team to notice any stealthy players trying to flank you from certain positions. Grenade damage through walls is no longer so annoying that you'll rage quit all the time, the same goes for bullet damage through really thick walls that still seem to hit you. A lot of this paragraph is research as I wasn't a big Counter Strike 1.6 player, but there important aspects of the game that effect how you play.
Source also includes Half-Life 2's physics system, which I much say has made for some very hilarious moments. The physics of the source engine are realistic when they need to be, providing added realism for normal game modes such as the classic modes of defusing and planting etc... but clever community members use the physics wisely in created game modes. A popular game mode "Deathrun", in which you manoeuvre through various obstacles whilst a terrorist has control of the switches for the traps, using the physics system to humorously drop things on your head, throw spikes up at you, or catapult you thousands of feet into the air with smoke as you shout out yourself wondering "why did I not see that coming?!".
Counter Strike Source was not envisioned by Valve as "Counter Strike 2". As a result it doesn't completely over-change anything to make it seem like a complete reboot, it's merely a faithful, respectable and extremely enjoyable re-skin of the ol' faithful original. Which is not a bad thing at all, it's as near to perfection as counter strike can possibly be. I believe Valves intentions for Source were purely to polish and shine the original game everybody knew, 1.6 wasn't a broken mess of a game, so there was no need to fix it. As they say, "If it ain't broke don't fix it".
Despite the problems for newcomers to the game, Counter Strike source is a well polished, enhanced version of a great game, and anyone who owns a PC should have this game installed, period.