You won't be able to make praetorians, do the new incarnate stuff, or have characters switch sides without going rogue.
Not sure on the specifics, but that's the gist of it.
Characters!:
Pinny - Scrapper
Shadewing - Defender
@Pinny
There are some costume pieces you might see and want... yet can't...
Will not be able to take full advantage of the alignment system/tips.
Most of what you will be missing, since you don't care about the 1-20, happens at 50. Without going rouge you won't be able to participate in the "endgame" Incarnate system. Only the initial slot, Alpha, is available, but with issue 20 there will be 4 more. The content that comes along with the Incarnate system are two lvl 50 Alpha TF/SFs, and with issue 20, the numerous Incarnate Trials.
I'd say you can get by without it just fine, for now. Just depends on what is enjoyable for you in the game. Getting to 50 is only part of the ride for me, it's only just beginning 
I would decide before Issue 20 hits...
Issue 24 PPM Calculator // The Great Makeover: The Vindicators
Welcome back!!
Yeah, maybe a few costume pieces and, under level 50, mainly just the Tip Missions.
Once you are level 20 and higher, you'll randomly/occasionally loot a Tip and that appears in your contact menu and you click on it, read it and do a one-of mission from it. Besides its impact in the whole Alignment Progression (You get two choices from reading the mission tip... One will lead you towards Heroism and the other Villainy [and their greyer in-betweens, sort of]), it's a little something different to do as you move along (and doing a bunch of them leads to getting Alignment Merits, which you can use to acquire specific Invention Recipes you might want).
I think you'll be happily surviving without it, based on your mindset and interests.
If/when you do want to pick it up, you should be able to find the Complete Edition somewhere for a good price which will include a Month's Play ($15 value), plus it has some extra costume bits, auras and emotes (all available from level 1 and up).
Hope you enjoy coming back!
@Zethustra
"Now at midnight all the agents and the superhuman crew come out
and round up everyone that knows more than they do"-Dylan
and round up everyone that knows more than they do"-Dylan
Thanks everyone those were helpful answers and much of what I expected which is good thing. Eventually I want to mess around with new 1-20 content, costumes, etc. I just wanted to be sure I wouldn't be a step behind everyone while I'm leveling up. I'm sure I'll give in and buy it soon enough just wanted to be sure. Thanks again!
Hey everyone I've always been more of a forum lurker than poster but I'm coming back from my break I took as I awaited GR. I ended up staying gone longer than expected and just resubscribed yesterday.
What I really want to ask though is how much will I miss playing without GR for the time being? I want to play awhile before I commit to buying it and since I'm playing a PB I'm not really worried about the new 1-20 content. But what are some of the other things I'll currently miss out on while leveling up if any?
Thanks in advance for any helpful answers and hope to see everyone in game.