I think that all the attributes marked as system (or is_user_defined = 0) since you cannot remove them from the Default attribute set.
Here is a list of the attribute codes that fall in this category. There are quite a few.
name
description
short_description
sku
price
special_price
special_from_date
special_to_date
weight
meta_title
meta_keyword
meta_description
image
small_image
thumbnail
media_gallery
old_id
group_price
tier_price
news_from_date
news_to_date
gallery
status
url_key
url_path
minimal_price
is_recurring
recurring_profile
visibility
custom_design
custom_design_from
custom_design_to
custom_layout_update
page_layout
category_ids
options_container
required_options
has_options
image_label
small_image_label
thumbnail_label
created_at
updated_at
country_of_manufacture
msrp_enabled
msrp_display_actual_price_type
msrp
enable_googlecheckout
tax_class_id
gift_message_available
price_type
sku_type
weight_type
price_view
shipment_type
links_purchased_separately
samples_title
links_title
links_exist
You can get the list of these attributes from this query
SELECT
e.attribute_code
FROM
eav_attribute e
WHERE
e.entity_type_id =
(SELECT
entity_type_id
FROM
eav_entity_type
WHERE entity_type_code = 'catalog_product'
)
AND e.is_user_defined = 0