Primarily the two river gorges their tanks have too drive through to get into the town from the northwest.
There is a paved road from the northwest that comes right into Tbilisi.
The main factor that stopped them probably was the 5 Presidents from Eastern Europe.
The five Presidents have come and gone.
Russia was not quite ready for war there at this time.
I believe Russia does have the troop strength and the equipment to drive into town and take Saakasvili.
The president of Estonia is home but had this to say today -
I believe the roads run along side the rivers which are in gorges. At least that is what I saw in the videos from the region. Also appears that way on the map. The main thing about that is the tanks would have to enter in a single line with mountain walls on either side of the road/river. The worse place for tanks if they are fighting ground troops.