I think it would be the aura of authority. The istari came in the bodily clothing of old men, and while Pippin had, on his travels, been in the presence of elf lords and young men of high rank, the older men (or seeming men) he had encountered were Gandalf, Theoden (revived), Saruman and Denethor. Now superficially Gandalf might have appeared the least imposing of them all. Denethor in his imposing surroundings and with the the ancient authority of his office would have been very imposing, and Denethor is very learned - it just occurs to me - and of course at this stage Pippin is unaware of Denethor's palantir which is an obvious parralel - that Pippin may be reminded of Saruman in his first impressions of Denethor. If I remember rightly the Steward has a rod as a sign of his office which would be a surrogate staff? I will have a closer look at that possibility when I reread...
