The Torah states that certain Zekeinim, elders, "saw" Hashem at Har Sinai (שמות כ"ד:י"א). The Rashbam (שם) says that seeing Hashem is a necessary part of a covenant. Whenever a covenant is formed, Hashem appears. For example, when Hashem and Avraham establish the Bris Bein HaBesarim, Hashem appears (בראשית ט"ו:י"ז); likewise later in the Torah, Hashem passes before Moshe to establish a covenant (שמות ל"ד:ו'). So too here, Hashem passed before the Zekeinim to establish a covenant. The Posuk in our Parsha (שמות כ"ד:י"א) relates that seeing Hashem in no way harmed these Zekeinim. However, in other places, seeing Hashem did cause harm. For example, in Sefer Shemuel Aleph (ו':י"ט), we read that the people of Beis Shemesh were harmed "because they had looked into the Ark of Hashem." Even so, Hashem doesn't harm the Zekeinim here.
After the vision, the Torah (שם) says, the Zekeinim held a great celebration at which they ate and drank with great joy. Ibn Ezra (שם) explains that this shows the different level that they where on, compared to Moshe. Moshe was on Har Sinai for forty days and did not need to eat or drink; the Zekeinim however, had to eat and drink after seeing Hashem in order to survive. Rabbeinu Bechaya explains that the Zekeinim held a great celebration after seeing Hashem in honor of their having survived the experience. He explains this as being similar to the great party which the Kohein Gadol would hold on Yom Kippur after surviving going into the קדש הקדשים, the holy of holies, where Hashem's Shechinah rests; the party was to celebrate his survival. The Seforno explains the Zekeinim's encounter with Hashem as Ibn Ezra does, explaining that they achieved a very high level of prophecy without losing their senses. Both King Shaul and Yechezkel lost their senses after an encounter with Hashem, but the Zekeinim were fine and were able to even eat and drink. The Torah thus wishes to stress the achievement of this high point of prophecy.
Rashi, however, understands this whole idea negatively. He says that the Zekeinim ate and drank in a vulgar manner after their encounter with Hashem, scorning the status they had achieved and that they deserved death for this. But Hashem did not want to disturb the joy of the covenant with their deaths, so He waited to punish them until later. The instant death that came to Nadav and Avihu described later in the Torah (ויקרא י':ב') was in part a punishment for this act. The Rambam explains similarly in his Moreh Nevuchim (חלק א' פרק ה') that the Zekeinim perceived Hashem in a physical manner after their encounter with His presence. Through this thought, they themselves became physical and had to eat and drink, and, as Rashi explained, Hashem waited until later to punish them.