Hm... I actually would say its major impact is actually at high levels. at 16 you have TWO level 8 spell slots. Thats Mass Heal and... Mass Heal.
Clerics did obtain new cleric only spells however, Radiant Assault, Lion's Roar, and Withering Palm are all cleric only I think.
PnP Clerics are more powerful than PotM clerics now, there is no doubt about that, to me. EO did say that he was working on tweaks to the domain system though, I reserve real impressions til then, it may not feel like as much of a slot loss if the domains actually make different clerics feel more unique in spell selections. Lots of the new non-cleric spells fit thematically well with domains or are actually ON the domain spell lists and with a motivated look at the Domain system it could be a huge net benefit imho. Disintegrate in in game now, its a Destruction Domain spell. Anger of the Noonday Sun is perfect for Sun domain, Coma and Deep Slumber is great for the Sleep domain, Halt Undead would be a good choice for Repose or Undeath etc. I think the spell selections allow a for clear divide between the Water Domain and potential Cold domain addition too. While Water could still have Ice Storm, Cone of Cold etc.... a potential Cold domain could have Freeze and Creeping Cold etc.
I do think the spell addition selections left the cleric spell list as an after thought, especially at the lower levels, but with the slot change, its doubtful anyone would find real use for lower level slots for anything other than keeping up with the party's needs as they go. You are better off either going a heavy armor and shield using melee cleric or as high of a Wisdom as you can go spell casting cleric, if you find yourself someone in between the two extremes you will feel the spell slot loss more than others. It never really pays to be well rounded in NWN, so nothing new.
Spiritual Weapon, Wave of Grief, Curse of Ill Fortune.... these are all spells that could have been added to the clerics level 2 spell selection for example. You think anyone would use them with 2 slots at level 3, 3 slots at level 4 etc?
No way that anyone would use those over Bull's Strength or Eagles/Owls/Bears etc.
Clerics lack.... viable debuffing spells at all levels with I think the noted exception of Prayer, lack a useful summon, have no damage shield before Aura against Alignment, and I struggle to name a touch spell worth the risk of a spell slot other than the Inflict spells. I would have liked these gaps addressed, if only to see clerics do something other than buff self and wear armor or just be the team medic, but I remain in awe of the amazing additions that we did get.
What I do like about the new spells, even the ones shared with Wiz/Sorc list... is the fact that they offer some alignment neutral alternatives to the obviously good or evil spells that made up the spellbook before at higher levels. Lucent Lance, Radiant Assault, Lion's Roar, Ice Flowers are very alignment friendly, as light and sound waves don't choose sides