Things of note for Free vs. Subscription:
1) Archetypes vs. Freeform. This is a very in-depth topic, but the big take away is that if you refuse to pay a single dime to Cryptic, you're stuck picking from about 10 different builds. Those are fixed, rigid builds, with a minimum of choice in terms of powers, team roles, and ability to specialize. Gold members can play any of the archetypes without paying extra for them if you find Freeform a little daunting. (I don't currently know if you get to keep playing archetypes if you lose Gold status, although the documentation I've read seems to imply that you do. That seems reasonable given that the subscription fee is higher than the cost of unlocking an archetype.)
2) Character Slots. I don't remember the exact number, but you get like 6 to 8 character slots as a Gold member versus the 2 you get for free. In addition, if you're a Gold member and you get a character to level 40, you get a new character slot for free. So as long as you're a subscriber, you can't fill your slots without going alt-crazy. (That's small consolation for some of us, but it's there.)
3) "Free" Zen. Just like you got some Paragon Points for being a subscriber in CoH, you get a supplementary supply of Zen each month. It's not huge, but it's enough to Total Retcon a character or get a costume unlock or two.
4) Locked Content. I haven't run into any content that's available to Gold Players but not to Silver Players yet, but that may be more out of elegant design than a lack of Gold content. That being said, when new Comic Series content (think Signature Story Arcs) are released, they stay Gold-only for a while. There's also some end-game content not open to Silver players, although I haven't hit level 40 yet so I can't comment on what's there and what isn't and what you'd be missing.
Points 1 and 2 are the big ones, in my opinion. Freeform characters (or even just some of the premium archetypes) are way more interesting than the strictly-free stuff, and I think I only know four gamers ever who could be perfectly happy with two character slots forever. Having said all that, there's an in-game currency called Questionite which you can collect doing various daily tasks which you're allowed to trade for Zen, albiet at a steep rate. (Currently, 200 Questionite gets you 1 zen, which is equal to one penny. Players set that rate, by the way, it's a market, not a vendor.) You're capped to "refining" 8,000 Questionite per day, even if you can manage to earn more, but A) that's per character and B) I've never actually hit the daily cap anyway. It takes a while to get enough of the stuff to trade for enough Zen to purchase something like a character slot or a new Archetype, but it can be done with patience. (Like, if you hit the refinement cap on two characters every day, which would take several hours at best, it would still take you two full weeks to get enough Zen to purchase 2 new character slots. In reality, that's more likely to take a month or two.
So, yeah, it's either put in time, or put in money, or be happy with a bare-bones game and some cool people to play it with. Pick one.
