Many SLA researchers agree that the implicit linguistic system, the mass of the iceberg below water, is developed ONLY via input. A comprehension-based approach builds the iceberg from the bottom-up. Rule-based instruction, with its focus on grammar presentation and output practice, focuses on the tip. Whatever tip rule-based instruction manages to construct is quick to melt away.

“Consciously learned grammar is only available as a Monitor or an editor, and the constraints on Monitor use are severe: 
1. The user has to know the rule, 
2. have time to apply the rule, 
3. and be thinking about correctness.” 
- Krashen

“Instruction does not appear to influence the order of development. No matter what order grammatical structures are presented and practiced in the classroom, learners will follow their own 'built-in' syllabus.” – Ellis

The Frequency Flower: The outer circle represents the language whole. The further from the center, the lower the frequency of that language structure.

A frequency-based approach starts with the innermost yellow disc and expands, each successive disc incorporating the previous disc, thus naturally ensuring frequent review. Vocabulary learning is "horizontal" (situational sets of words) as opposed to "vertical" (semantic sets - related words of the same part of speech).

Typical textbook syllabi cover (not necessarily learn) the petals one at a time, with its focus on vertical vocabulary lists (e.g., colors, body parts, foods). These semantic sets contain many mid and low frequency words. There is much less recycling of the language and not all of the petals get covered, creating gaps in useful language. 

50 words = 60% of speech 
1,000 words = 85% of speech
Verbs = 20% of the language

Examples of the highest frequency and lowest frequency thematic words traditionally taught:

Why do we care so much that someone can count to ten or higher?
(72)one (56)two (119)three (188)four (231)five (375)six (470)seven (492)eight (786)nine (346)ten

Only the numbers 1 and 2 are in the most-frequently used 100 words. There are 300+ more frequent words than the numbers 6 through 10, and the numbers 13 through 19 are not in the most frequently used 1,000 Spanish words.
Source: A Frequency Dictionary of Spanish (2006) by Mark Davies