I had that impression too when I first pulled the trigger, but after playing through about 75% of the campaign, I think you (and the earlier me) are selling it short.
There are two types of covert missions, but skads of new maps unique to those missions, along with the global panic-mechanics.
There are at least five “one-off” EXALT missions (plus the XCOM base attack) that I’ve counted so far, and I suspect I’m missing one or haven’t seen some yet:
EXALT Mission List
The initial one where you find out that there is an EXALT
The one where you capture/rescue the EXALT guy
The one where you rescue the girl
The XCOM base assault
The assault on the EXALT base
The rescue mission for the other abductees
… that’s pretty much the equivalent of all the scripted missions in the EO original and doesn’t even include the additional one-offs for the campaign, like the fishing village.
And come to think of it, there are so many EXALT one-off missions that they kind of crowd out the generic EO “council” missions all together; I think in the EW play-through I’ve only gotten one or maybe two non-EXALT council missions, where in the typical EO play-through I’d start to get tired of the repetition. I don’t know if that’s good or bad, but I don’t miss trying to rescue yet another council flunky from that same damned ruined highway yet again.
There at least two new alien types, plus you technically get five or six new enemies in the various EXALT troop types. I know the latter might initially seem like a trivial addition, but I disagree: in the long late-mid game of EO, I got REALLY tired of shooting the same damned Mutons again and again, mission after mission. The EXALT missions give you some variety, and I like how their skills and weapons upgrade alongside yours.
You get the new MEC trooper, the MECs themselves and their weapon/armor upgrade paths.
You get the new genetic engineering stuff, which is effectively a new opt-in skill tree.
You get the new MELD resource and its associated mechanics in most missions.
47 new maps – basically increasing the “generic” maps by 50% plus the one-offs.
A bunch of new weapons, grenade types, accessories and armor, even beyond the new stuff associated with the MECs.
The medals mechanic, which at first seems pretty trivial and annoying, but which scales pretty nicely and makes your officers more survivable.
Plus there are the (relatively minor) UI and skill tweaks, the soldier language stuff, the new faces and helmets, etc., etc.
That’s a pretty decent list of additions for an expansion, at least in retrospect.