A big mistake Cryptic made is in the speed of levelling.
The average Cryptic MMO is quite fast in levelling, but STO takes it to 11.
Combine that with the fact that it also takes the "little in terms of endgame content", another Cryptic staple, to 11 as well and you've got bored players.
You can tell bored players you were rushed, but in the end they're still bored.
The funny thing is that I personally like the fast leveling myself..but I do know that I am in minority on that one.
These days I basically level up my new toons to lvl 50 before I start making the FEs and so on.
That's because you are a veteran.
The whole point of a leveling experience is to learn the gameplay mechanics step by step which doesn't happen when you are at max the day after you started.
If the leveling curve becomes less demanding and faster when you run alts that would be ok.
Let's say you level your first toon and reach max level at some point, from that point on every bit of experience he earns gets added to an account XP pool. That pool would be used to add 20% to the XP of a new toon and could add up with XP boosts from C-Sto... sorry, ZEN Store, or you could use that extra XP to buy special leveling ship variants or kits and other gear.
It would be similar to but different enough from SWTOR's legacy system and encourage players to run alts and spend on the store.